Are you using any code or any other way to create a collection?
Below are some general reasons :
1) Might be you reached the maximum number of authorized
collections for an account is 10, Please verify and confirm back.
2) Make sure that you are not using the same collection name with existing one.
If you are facing the same issue after following the above points, then it may be due to some connectivity errors. Try to login on portal after some time.
Please let us know for further assistance.
Regards,