uk area code issue

The UK web page for Apple Trade In: Apple Trade In - Apple (UK). Verify that you don’t use the Trade In page meant for other countries, if that doesn’t apply to you.

You seem to be inquiring about the form entry of a phone number. The first digits after the trunk prefix of 0 (zero) is the area code of the phone number (also called area code for mobile, even though without geographical meaning). So, if your number is e.g. 0 7890 123 456, then “7890” would be the area code.

As the formatting of phone numbers is country-dependent, please verify that you use a form for the same country as what the phone number is for. A mismatch wouldn’t work. I can’t think of other reasons. Also try settings the cursor in a different field to possibly re-evaluate the validity of the last form field entry. Else contact Apple Support for more hands-on support.

it still just says your entry is not formatted correctly underneath the area code

Then you must be on the US site - we use zip codes. So, in the US, it works with zip codes. Trade in is not available in all countries/regions. Best check in your region.