By default, all users can enter time to all active projects provided you have the relevant subscription package and security permissions. There are only a few things that would prevent you from doing this:
Manage Access has been implemented and that project is not assigned to you.
Project assignments have been implemented (Projects > General > Assignment) and you are not assigned to that project.
The 'Prevent time entry for the project' rule has been applied to that project (Projects > General > Assignment). In this case, no one can enter time to the project.