Unable to alter table - reason code is 10
Thanks to Charles for pointing out this command: DSPFD AMMLIBT/SAGRPPF

Which showed me that:


  Maximum members . . . . . . . . . . . . . . : MAXMBRS    *NOMAX


To fix this, I needed to change '*NOMAX' to '1'. To do this, I went to my greenscreen and did:


type chgpf ammlibc/sagrppf
F4
F10
Move cursor to 'Maximum members'
Change '*NOMAX' to '1'
Enter