It's true that your Account Type is eligible for unlimited storage. However, Box Administrators can still limit their managed user's account settings including the user's storage allocation.
Upon checking, what prevents you from successfully uploading files. Is that you've already reached your storage limit and you can reach out to your Box Administrator to increase your storage.
To check your storage:
Log in to the Box website and navigate to your Account Settings.
We need a bigger allowance for our storage. i stated this clearly when i speak to an agent and he never mention about this issue. How are we going to have bigger storage in this case.