1 Answers
During the trial period, you have somewhat a quota of "Billing Account Linking Quota", i.e. you can only link a certain number of projects to your Billing Account. The quota can be 3 or 5 or maybe something else (in my case it’s 5).
If you suddenly cannot enable Billing on any new project, it’s likely that you’ve exceeded this so-called quota. You may want to unlink some existing/unused project so that new projects can be linked.