I just had this bug happen to me and it's very clear. If you first assign a Global variable to one data type and then change that data type you will definitely get this error. "Assessment" is a SharePoint list and originally it was a Choice field and then I changed it to a Single Line of text. The fix was simply to rename the variable to "varCurrentQuarter" from "varQuarter". So somewhere inside PA's is typing this variable and not letting you change it.