Hi - I had a problem trying to input my imei last year when trying to claim my free watch 4 - kept saying invalid. However there was also an option on Samsung's claim site to upload a photo of the imei number on the box my phone came in, and a screenshot of the imei number on the phone if it wouldn't accept the number you keyed in ( my claim all worked OK with doing that ) Might be worth re checking the trade in offer site to see if that is an option anywhere. Good luck with it
I had a problem with a Galaxy S22. I could attach a photo of the IMEI. Then it was processed without any problem. I sent an emails to tradeinresponse on something else last week and got a fast response. So sure they will contact you.