Selected Answer
Marcco
I don't get any error when I press the Add Item button on the Input worksheet of the file you attached (which needs to be removed please- see my Discussion point below your question). It correctly adds the Qty* Price formula to cel F14, F15 etc. as items are added.
You have the named ranges defined and sheet names correct so I can't see a cause for such an error.
I can force worksheet #VALUE! errors by putting text in the Qty and Price fields (or using a comma as decimal separator where mine is the fullstop/ period). No error 1004 though.
Revision 22 March 2022:
The failed line is:
Worksheets("Invoice").Range("subtotalRow").Offset(-2, 1).Formula2R1C1 = "=RC[-1]*RC[-2]"
That works in later versions of Excel (365, 2021, Excel for the web) which support Dynamic Arrays (which make very easy to enter an array formula e.g.) but not earlier versions..
Given you use Excel 2019 Pro+, remove the 2, so:
Worksheets("Invoice").Range("subtotalRow").Offset(-2, 1).FormulaR1C1 = "=RC[-1]*RC[-2]"
Hope this fixes things for you. If so, please remember to mark this answer as Selected.