AVERAGEIFS only works on open workbooks, so if the other file is closed, it will return an error.

AVERAGEIFs() also starts with the average-range, then lists the criteria range and criteria (unlike the regular AVERAGEIF) - looks like you have it misplaced

Also, you dont need to include "=" if it is a direct reference like that (its not wrong, just unnecessary)

=AVERAGEIFS(Mission!$A$1:$A$122,"="&C4,Mission!$C$1:$C$122,"<300")
you could just use...
=AVERAGEIFS(Mission!$A$1:$A$122,C4,Mission!$C$1:$C$122,"<300")