Okay, first one of your defined names (RepairNoDetails) had the range in Quotes (="RDetails[Repair '#]" should be =RDetails[Repair '#])
Modified your formula in Repair!B5 as follows
WAS
=IFERROR(INDEX(RDetails,SMALL(IF(RepairNoDetails=rngRepair,ROW(RDetails)-ROW(RDetails[#Headers])),ROW(1:1)),MATCH(Repair!B$4,RDetails[#Headers],0)),"-")
It also needs to be entered as an array function. Enter with CNTRL SHFT ENTER instead of ENTER. If done properly, you'll see {} around your formula.
See attachment
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