Does anything here help?
https://unitopten.com/fix-barclays-contactless-mobile-payments-issues/
(If NFC is already turned on, try turning it off and then on again. Also try enabling Flight Mode and then disabling it).
Try opening the Barclays app to see if there's an 'Update your settings' message there. (If there is then, obviously, follow the procedure to do so). Then, irrespective of whether the message was there or not, LOG OUT of the app. (DON'T just close it). Oddly, there's no option to do so from the Home screen. You have to tap your account balance, to view your transactions, and then tap the three dots at the top right. Then log back in again. (If you've updated the settings, you'll then be asked to accept the terms and conditions. Do so). Then log out again. That often fixes problems with the Barclays app (although it can sometimes take several minutes for the 'Contactless' symbol to appear).
Otherwise try going to the app and, rather than typing in your PIN to access it, tap 'Contactless mobile' at the bottom left of your screen. You can then enter your debit card PIN (rather than the app one) to make a contactless payment.
Getting the 'contactless' feature on the Barclays app to work though never seems to be easy. For me, the 'Contactless' icon always seems to be there - EXCEPT when I actually want to make a contactless payment!