Also called BT-130, unit of measure code, UN/ECE Recommendation 20
Every EN 16931 invoice line states its quantity in a unit taken from a fixed code list, UN/ECE Recommendation 20. The codes are not intuitive: a plain count of things is C62, hours are HUR and kilograms are KGM.
C62 is "one", the code for a plain countable item. It is the right answer far more often than anything else and it is the least guessable code in the list.
HUR is hours, which is what most service invoices need. KGM is kilograms. DAY is days, MTR is metres, LTR is litres.
What does not work is the word. "each", "hours" and "pcs" are not codes, and a line carrying one of those fails a code list rule rather than being interpreted charitably.
The rule that fires is a BR-CL code list rule, and its message says the value is not in the permitted list without saying which list or what a valid entry looks like.
So the error reads as though something exotic is wrong when the fix is usually to send C62 instead of the word "unit".
Every BR-CL rule, what causes it and what to send instead.