Ok, try:

=IF(ISNUMBER(MATCH(I5,FSR_NGP!$F:$F,0)),INDEX(FSR_NGP!$C:$C,MATCH(I5,FSR_NGP!$F:$F,0)),INDEX(fsr_akl!$C:$C,MATCH(I5,fsr_akl!$F:$F,0)))