If you want to join data from different files, load a dummy table before For Each loop. Then join will work.
Load 0 as Dummy
For Each file in filelist('pathtoFiles\*.xlsx')
let findfield = FieldNumber(Denial, '$(file)'); //Added semicolon
if $(findfield) > 0 then
Load * from file where Denial = 'Yes';
endif //Removed semicolon
next file //Removed semicolon
Drop Field Dummy From Table1; //Drop the dummy field
Try this and let us know the result.