I’m facing same issue. It started with activation of 2FA authentication on gitlab. I removed Cloudflare token from gitlab and set integration once again to force authorization token generation and it worked. Well at least for couple of days, because today i have the same error again on each project. I will turn off 2FA in gitlab and check again.
I’ve checked it once again and it works. Go to gitlab: https://gitlab.com/-/profile/applications and remove Cloudflare token. Go back to to Cloudflare → Pages → Create repository → Connect to Gitlab. Once you authorize cf in gitlab you can close wizard and came back to project and fire build again.