While linking my 4th Project with my trial billing account, getting an error saying that "You have reached the limit of projects on which you can enable billing. You can request an increase in your billing quota if you need to enable billing on additional projects."
There’s an "invisible" soft limit of the amount of projects that you can enable billing during the free trial period. I said "invisible" because I don’t see that is mentioned anywhere when registering a free account for trial usage (or there is but I don’t know where :D). This limit seems to be fluctuated between 3 and 5 and it’s different from account to account. For me personally, it was a 5-projects limit that I can enable billing on.
Just disable billing on an unused project so you have a slot for new project to come in.