Amforth on Arduino UNO
I programmed by Uno using your first guide and it seems to work OK.

amforth 6.1 ATmega
8P Forthduino


For example, using the serial monitor, I entered:

6 7 * .


It printed:

42  ok
&gt; 


Then  entered the ASCII table example:

\ print a ascii table
: .ascii
base @
$7f $20 do
    i emit
    decimal  ." , dec: " i .
    hex      ." , hex: " i .
    8 base ! ." , oct: " i .
    2 base ! ." , bin: " i .
    cr
loop
base !
;


Then when I invoked it with ".ascii" it printed the table:

&gt; .ascii
 , dec: 32 , hex: 20 , oct: 40 , bin: 100000 
!, dec: 33 , hex: 21 , oct: 41 , bin: 100001 
", dec: 34 , hex: 22 , oct: 42 , bin: 100010 
#, dec: 35 , hex: 23 , oct: 43 , bin: 100011 
$, dec: 36 , hex: 24 , oct: 44 , bin: 100100 
%, dec: 37 , hex: 25 , oct: 45 , bin: 100101 
&amp;, dec: 38 , hex: 26 , oct: 46 , bin: 100110 
', dec: 39 , hex: 27 , oct: 47 , bin: 100111 
(, dec: 40 , hex: 28 , oct: 50 , bin: 101000 
), dec: 41 , hex: 29 , oct: 51 , bin: 101001 
*, dec: 42 , hex: 2A , oct: 52 , bin: 101010 
+, dec: 43 , hex: 2B , oct: 53 , bin: 101011 
,, dec: 44 , hex: 2C , oct: 54 , bin: 101100 
-, dec: 45 , hex: 2D , oct: 55 , bin: 101101 
., dec: 46 , hex: 2E , oct: 56 , bin: 101110 
/, dec: 47 , hex: 2F , oct: 57 , bin: 101111 
0, dec: 48 , hex: 30 , oct: 60 , bin: 110000 
1, dec: 49 , hex: 31 , oct: 61 , bin: 110001 
2, dec: 50 , hex: 32 , oct: 62 , bin: 110010 
3, dec: 51 , hex: 33 , oct: 63 , bin: 110011 
4, dec: 52 , hex: 34 , oct: 64 , bin: 110100 
5, dec: 53 , hex: 35 , oct: 65 , bin: 110101 
6, dec: 54 , hex: 36 , oct: 66 , bin: 110110 
7, dec: 55 , hex: 37 , oct: 67 , bin: 110111 
8, dec: 56 , hex: 38 , oct: 70 , bin: 111000 
9, dec: 57 , hex: 39 , oct: 71 , bin: 111001 
:, dec: 58 , hex: 3A , oct: 72 , bin: 111010 
;, dec: 59 , hex: 3B , oct: 73 , bin: 111011 
&lt;, dec: 60 , hex: 3C , oct: 74 , bin: 111100 
=, dec: 61 , hex: 3D , oct: 75 , bin: 111101 
&gt;, dec: 62 , hex: 3E , oct: 76 , bin: 111110 
?, dec: 63 , hex: 3F , oct: 77 , bin: 111111 
@, dec: 64 , hex: 40 , oct: 100 , bin: 1000000 
A, dec: 65 , hex: 41 , oct: 101 , bin: 1000001 
B, dec: 66 , hex: 42 , oct: 102 , bin: 1000010 
C, dec: 67 , hex: 43 , oct: 103 , bin: 1000011 
D, dec: 68 , hex: 44 , oct: 104 , bin: 1000100 
E, dec: 69 , hex: 45 , oct: 105 , bin: 1000101 
F, dec: 70 , hex: 46 , oct: 106 , bin: 1000110 
G, dec: 71 , hex: 47 , oct: 107 , bin: 1000111 
H, dec: 72 , hex: 48 , oct: 110 , bin: 1001000 
I, dec: 73 , hex: 49 , oct: 111 , bin: 1001001 
J, dec: 74 , hex: 4A , oct: 112 , bin: 1001010 
K, dec: 75 , hex: 4B , oct: 113 , bin: 1001011 
L, dec: 76 , hex: 4C , oct: 114 , bin: 1001100 
M, dec: 77 , hex: 4D , oct: 115 , bin: 1001101 
N, dec: 78 , hex: 4E , oct: 116 , bin: 1001110 
O, dec: 79 , hex: 4F , oct: 117 , bin: 1001111 
P, dec: 80 , hex: 50 , oct: 120 , bin: 1010000 
Q, dec: 81 , hex: 51 , oct: 121 , bin: 1010001 
R, dec: 82 , hex: 52 , oct: 122 , bin: 1010010 
S, dec: 83 , hex: 53 , oct: 123 , bin: 1010011 
T, dec: 84 , hex: 54 , oct: 124 , bin: 1010100 
U, dec: 85 , hex: 55 , oct: 125 , bin: 1010101 
V, dec: 86 , hex: 56 , oct: 126 , bin: 1010110 
W, dec: 87 , hex: 57 , oct: 127 , bin: 1010111 
X, dec: 88 , hex: 58 , oct: 130 , bin: 1011000 
Y, dec: 89 , hex: 59 , oct: 131 , bin: 1011001 
Z, dec: 90 , hex: 5A , oct: 132 , bin: 1011010 
[, dec: 91 , hex: 5B , oct: 133 , bin: 1011011 
\, dec: 92 , hex: 5C , oct: 134 , bin: 1011100 
], dec: 93 , hex: 5D , oct: 135 , bin: 1011101 
^, dec: 94 , hex: 5E , oct: 136 , bin: 1011110 
_, dec: 95 , hex: 5F , oct: 137 , bin: 1011111 
`, dec: 96 , hex: 60 , oct: 140 , bin: 1100000 
a, dec: 97 , hex: 61 , oct: 141 , bin: 1100001 
b, dec: 98 , hex: 62 , oct: 142 , bin: 1100010 
c, dec: 99 , hex: 63 , oct: 143 , bin: 1100011 
d, dec: 100 , hex: 64 , oct: 144 , bin: 1100100 
e, dec: 101 , hex: 65 , oct: 145 , bin: 1100101 
f, dec: 102 , hex: 66 , oct: 146 , bin: 1100110 
g, dec: 103 , hex: 67 , oct: 147 , bin: 1100111 
h, dec: 104 , hex: 68 , oct: 150 , bin: 1101000 
i, dec: 105 , hex: 69 , oct: 151 , bin: 1101001 
j, dec: 106 , hex: 6A , oct: 152 , bin: 1101010 
k, dec: 107 , hex: 6B , oct: 153 , bin: 1101011 
l, dec: 108 , hex: 6C , oct: 154 , bin: 1101100 
m, dec: 109 , hex: 6D , oct: 155 , bin: 1101101 
n, dec: 110 , hex: 6E , oct: 156 , bin: 1101110 
o, dec: 111 , hex: 6F , oct: 157 , bin: 1101111 
p, dec: 112 , hex: 70 , oct: 160 , bin: 1110000 
q, dec: 113 , hex: 71 , oct: 161 , bin: 1110001 
r, dec: 114 , hex: 72 , oct: 162 , bin: 1110010 
s, dec: 115 , hex: 73 , oct: 163 , bin: 1110011 
t, dec: 116 , hex: 74 , oct: 164 , bin: 1110100 
u, dec: 117 , hex: 75 , oct: 165 , bin: 1110101 
v, dec: 118 , hex: 76 , oct: 166 , bin: 1110110 
w, dec: 119 , hex: 77 , oct: 167 , bin: 1110111 
x, dec: 120 , hex: 78 , oct: 170 , bin: 1111000 
y, dec: 121 , hex: 79 , oct: 171 , bin: 1111001 
z, dec: 122 , hex: 7A , oct: 172 , bin: 1111010 
{, dec: 123 , hex: 7B , oct: 173 , bin: 1111011 
|, dec: 124 , hex: 7C , oct: 174 , bin: 1111100 
}, dec: 125 , hex: 7D , oct: 175 , bin: 1111101 
~, dec: 126 , hex: 7E , oct: 176 , bin: 1111110 
 ok
&gt; 




I suspect your issue is you are not using it the way it is supposed to be used. Can you show the exact commands you type which cause the error?



(Edited to add)

After fairly extensive investigation of the source, I think that .s is not necessarily implemented for the Uno .hex file.

I found one suggested implementation here:

: .s  ( -- )\ stack picture listing order
depth
begin dup while dup pick . 1- repeat
drop
;


Running that it seems to work OK:

&gt; 1 2 3 4 .s
1 2 3 4  ok


The .s word is implemented in the file dot-s.asm where you can see exactly how they did it (it's not the same as the Forth version above).