All the other values work fine, but the STAB value, when the question asks if it's the same type as the Pokemon, doesn't seem to change from Yes or No to 1.5 or 1 respectively.

Here's the script:

ask [Is this the same type as the Pokemon? (Yes, No)] and wait
set [STAB v] to (answer)
repeat until <<(answer) = [yes]> or <(answer) = [no]>>
if <(answer) = [yes]> then
set [STAB v] to [1.5]
end
if <(answer) = [no]> then
set [STAB v] to [1]
end

And here's the project: https://scratch.mit.edu/projects/114305126/

Edit: I just removed the repeat until thing and now it works.

Glad you figured it out. I'll close this to prevent any errant responses, but if you need it re-opened you can report this and ask.