Check the code exactly as supplied, its expiry, minimum basket value, eligible products, region, account restrictions, and whether another promotion conflicts with it. Remove accidental spaces and confirm you are using the correct seller's checkout.
If the code still fails, do not keep trying random codes during a timer. Compare the delivered total without the discount with your target price and recent price history. Check stock, warranty, and return terms; a small discount is not worth a poorer seller. If the stated terms say the code should apply, document the error and contact the seller before paying.
The buying decision is about the final amount and terms, not the code itself.
Akousa