I am getting the same error, the AddColumns with Lookup works fine if I use the unfiltered source, but if I filter the source then the Lookup comparison complains the column on the right side (my filtered table) is returning a table type instead of the number type of a single record in that column. I suspect this means it is returning all the values of the column instead of just the current record's value. Were you ever able to get this working, and if so what was the solution?
Does not work:
ShowColumns(AddColumns(Filter('pbi.Students_byMonth', SY = Dropdown1.Selected.Result),"School", LookUp(LOC, CD = 'pbi.Students_byMonth'[@SC],NM)),"ID","School","SY")
But if I replace the lookup with a static value I can filter:
ShowColumns(AddColumns(Filter('pbi.Students_byMonth', SY = Dropdown1.Selected.Result),"School", 1),"ID","School","SY")
or if I don't filter the source I can use the lookup
ShowColumns(AddColumns('pbi.Students_byMonth',"School", LookUp(LOC, CD = 'pbi.Students_byMonth'[@SC],NM)),"ID","School","SY")
But trying to combine the two, a filter and a lookup, seems to need a bit more coaxing.