I think some keyboards have problems with “/” character in the Whonix installation.
Personally I solved this question using copy and paste.
In the instructions above form you can find a phrase like:
“bla- bla- bla- bla- bla… /24
So, just copy and paste “/24”
I am not sure I understand.
When you type
/
it prints some other char?
Perhaps you need to adjust key keyboard layout?