Trying to write data validation for if there is a certain value in AC column, a dropdown (that's based on a certain range) appears in AD column

[SkywardPalm]

Anything I try gives me the error "the list source must be a delimited list, or reference to single row or column"

What I have now is something like

=IFERROR(CHOOSE(IF($AC1="Update Location",1,IF($AC1="Update Source",2)),SUMMARY!$B$17:$B$226,Master!$X$2:$X$1048576),"")

where if Update Location is chosen in AC column, a list formed from unique values in range SUMMARY!$B$17:$B$226 appears, and if Update Location is chosen then a list from unique values in column X of the Master sheet appears..
Any advice would be greatly appreciated.