Just a thought - is ThisWorkbook the workbook where the code is -
and where Sheets("TR_template") is? May sound a silly Q, but if the
code is in a different workbook it's referencing the number of
sheets in a different workbook.

Good thought, but ThisWorkbook is the only one open, where the code
is, and where TR_template is. BTW, "add" works properly, but "copy"
does not.

OK - that excludes that, then!