Why can I cannot add or remove any files?

I can't do anything except view / review

It sounds like you were added to another person's project or account as a viewer/reviewer. If so, you will only be able to view/review within that project - whether or not you pay for your own account has no influence.

While you have a free account, you can still be invited as admin/member to someone else's paid account: E.g. if you need to participate actively in the project by uploading or managing projects and media, you will need to be added to this project/account as member or admin respectively.

Currently only paid accounts allow for more than one team member - this means whoever manages the project you need to work on needs to have a paid plan.

And if they don't yet, we offer a 14 day trial ;)

To learn more about accounts, projects, inviting users and their permissions click here.