If you notice after subscribing that one of your users is locked out (look for the lock symbol next to the user's name when visiting Account then Manage team from your left navigation), this means that you are beyond your maximum number of users for the plan you purchased. Locked out users cannot access your account.
To unlock a user you have 2 options:
Purchase additional users by updating your plan
Visit Account >> Settings from your left navigation
Select 'Manage your subscription'
Select 'Change your plan'
Update your plan to include the desired number of users
Once your plan has been updated to include a larger number of users, your locked out user will be given access to your account
Delete existing users to make room for the locked out user
If you would like to unlock a locked out user but keep the existing number of users in the plan you purchased, you can delete active users to make room for the locked out user.
To delete an active user:
Visit Account, then Manage team from your left navigation
Click on the 'delete' icon next to the active user's name
Check how many users are remaining in your account
You can check how many users are remaining in your account anytime by:
Visiting Account >> Manage Team from your left navigation
Next to "Manage Team" at the top of the page, see "X Users Remaining"