You're on the trial plan and are limited to 500MB Volumes. MongoDB can not happily run with only 500MB. You will need to upgrade to the Hobby plan for access to 5GB volumes. After the upgrade, go into the volume settings to grow it to 5GB.

Hi I have upgraded to the hobby plan even though I still can't grow the project 5GB. I think the issue is coming I used the free demo version and created the mongo db server there and after that I registered now that the demo version not identifying that I have upgraded the account. I used that demo server for a while thinking that was connected to my paid account but seems like it's not.