Go To Related records is intended to pull up a found set for a related set of records. These are usually, (unless you have a "self join" linking two occurrences of the same table), different tables so the current layout is probably not based on the same data source table as "Staff_tbl_TeacherEvaluationForm". And you wouldn't want to do that any way if you are going to use the Average aggregate function to compute the average. This calculation should evaluate from the context of the Parent record--the current record of the layout where you have a portal to Staff_tbl_TeacherEvaluationForm. If you tried to use it with this syntax on a layout based on the Staff_tbl_TeacherEvaluationForm, you would just get the value of Grade for the current record.
I suggest looking up this function in FileMaker Help or any training materials you might have to learn more about how aggregate functions work.