Seems like you are using a professional test account. Test account is made only for test purposes and has only 200 core hours. That's why you are having a qouta problem.
Although it is recommended to initially create an online free community account for the homework purposes. But no worries, our support engineer will be here to help you with this. He will change your account to the free community and you are good to go. All you have to do is to just send an email to him at: email@example.com (make subject of your email as computational qouta + your registered mail ID with SimScale in the message body)
For other users having this problem, please follow the same procedure.
After your account has been changed, you would be able to see 3000 core hours under your dashboard. This shows that the process was successful. But in order to further do modification in the same project, you have to make it public. In order to do so, please follow this forum post.
Hope this helps. If you have any question/s, feel free to ask.