You probably don't want to use curly brackets. Those are for a Level-of-Detail expression, which is a special kind of calculation.
This error message in Tableau ("Cannot mix aggregated and non aggregated argument to function") is common. It basically means, "Part of what you're doing is on the individual row level, but another part calculates across multiple rows (for example, with SUM). You have to pick one or the other."
I'm guessing that [Period] is from row-level records, but [AE Performance] is an aggregation (SUM, AVG, etc.).
What's the formula for [AE Performance]?