If it is an AV006 error then this workaround suggested to my by someone on HFP forum worked for me even though Barclays IT had said it was a known issue that could not be fixed and there was no way I could sign up to Avios rewards!

Go to the Barclays 100k promotion web page on your phone. Go through the eligibility checker screens. When you get to the question do you have Avios Rewards, answer no. This then takes you to the Avios Rewards sign up page in the app, bypassing the error message.

I hope this works for you, it did for me!