You can also just try to move a working version of the Monthly Budget from a fresh copy of the Foundation Template.
However, that may not actually help and could be some sort of config issue that’s breaking the Monthly Budget (e.g. are there any header keywords missing in row 1 on your Categories sheet?) FAQ guide below has a few things to check.
The other thing to note is that a Group and Type are required on the Categories sheet for each Category in order for the budget dashboards to work.