Try this version
=IF(OVERVIEW!$G$8>10,AVERAGEIFS('ratings data'!$D:$D,'ratings data'!$A:$A,">="&OVERVIEW!$C$2,'ratings data'!$A:$A,"<="&OVERVIEW!$C$8,'ratings data'!$C:$C, $B12),"--")
Numbers are not normally in quotes so I removed the quotes from 10
when you compare against a cell for a criterion in AVERAGEIFS
you need to concatenate like this
">="&OVERVIEW!$C$2
Should OVERVIEW!$C$8
actually be OVERVIEW!$C$3
as your text description says?