I would like to get an average percentage out of my sample, however, I need to use several conditions. I tried to use the AVERAGE and AVERAGEIF together with FILTER but everything returns an error and I think I'm incorrectly "merging" formulas.

You can find

or

=IFERROR(AVERAGE(FILTER(Data!N3:N,Data!B3:B=A3,Data!C3:C=B3,REGEXMATCH(Data!A3:A,"^0. "))),"")

or

=IFERROR(INDEX(QUERY({Data!A3:C,Data!N3:N},"select avg(Col4) where Col1 starts with '0' and Col2 = '"&A3&"' and Col3 = '"&B3&"'"),2,0),"")