Mysql error while creating FK
When you create a foreign key between two tables the data type of the columns that are taking part in the relationship must have the same data type. In the example you have posted above it is failing because the Categories table defines CatID as a BIGINT wheras in the Projects table you have defined it as a INT. Changing the data type of CatID in the Projects table to BIGINT should resolve your problem.

I hope this helps you.