Marie,
I try to explain it better:
Price Each = @xlookup(@Fn",Part Number,("Mastinv!Item number"),"Price Each")
Price Each is the present element in the subform
Part Number is the key in the subform to match.
Mastinv is the mainform
Item Number is the key in the mainform to match
Price Each is the element brought back from the mainform.
I worked on a xlookup one time for several hours, only to remember it only works in runtime.
Also had the problem with the security that Ray mentioned.
Done the one Bob mentioned too, misspelled element names, or bad syntax.
Good Luck !!