Merge not working

I keep getting the following error:
Query 'Merge2' (step 'Expanded Workbook(2)') references other queries or steps, so it may not directly access a data source. Please rebuild this data combination.

This is the merged query:
let
Source = Table.NestedJoin(#"Matching(3)", {"Unique ID"}, #"Workbook(2)", {"Unique ID"}, "Workbook(2)", JoinKind.RightOuter),
#"Expanded Workbook(2)" = Table.ExpandTableColumn(Source, "Workbook(2)", {"Unique ID", "What WG SFS Intervention level is intervention best suited to? (Dropdown) See note!", "What WG thematic outcome(s) does the intervention deliver? (Dropdown - select all that apply in descending order of delivery magnitude)"}, {"Unique ID.1", "What WG SFS Intervention level is intervention best suited to? (Dropdown) See no", "What WG thematic outcome(s) does the intervention deliver? (Dropdown - select al"})
in
#"Expanded Workbook(2)"

I have tried lots of different things. Looked at https://www.excelguru.ca/blog/2015/03/11/power-query-errors-please-rebuild-this-data-combination/  but can't get it to work.