How to convert a phylogeny to a dendrogram in R
For your example to work you should replace the rtree function with the rcoal function, see example below.

library(ape)  
library(dendextend)   
Tree &lt;- rtree(10, rooted=F)
is.ultrametric(Tree) 
[1] FALSE 
is.binary.tree(Tree) 
[1] TRUE

Tree_rcoal &lt;- rcoal(10, rooted=F) 
is.ultrametric(Tree_rcoal) 
[1] TRUE
is.binary.tree(Tree_rcoal) 
[1] TRUE

Tree_rcoal2 &lt;- rcoal(10, rooted=F)

tanglegram(Tree_rcoal, Tree_rcoal2)


To make use of a file in newick format, you can try to convert with the chronos function.

s &lt;- "owls(((Strix_aluco:4.2,Asio_otus:4.2):3.1,Athene_noctua:7.3):6.3,Tyto_alba:13.5);"
cat(s, file = "ex.tre", sep = "\n")
tree.owls &lt;- read.tree("ex.tre")
is.ultrametric(tree.owls)
[1] FALSE
dendrogram &lt;- chronos(tree.owls)

Setting initial dates...
Fitting in progress... get a first set of estimates
         Penalised log-lik = -24.04992 
Optimising rates... dates... -24.04992 
Optimising rates... dates... -24.04908 

Done.
is.ultrametric(dendrogram)
[1] TRUE