Most common cause of the symptom
You have ran out of seats on the project.
For Team plans, a maximum of 5 total users are permitted on each project. There must always be at least 1 Admin, otherwise, you can have any mix of quantities of Admin & Internal users which total up to 5.
On Enterprise plans, a bespoke maximum user limit is provided based upon your requirements. This can be any number from 1 user upwards. However, the maximum count on an Enterprise plan project does not include any external users.
There must always be at least 1 Admin, otherwise, you can have any mix of quantities of Admin, Internal & External users which total up to no more than the bespoke maximum amount provided. Once this limit has been reached, you will only be able to add new users to your project with the Read only role. See Add & invite users to your project.
1. Remove other users from your project to make room.
2. Enterprise plan only: Demote existing External users to Read only role to make room for more Admin, Internal or External users.
3. Get in touch with us at firstname.lastname@example.org to discuss increasing the limit on your plan.