Reactivating a CRM user...

Reactivating a CRM user...

I'm a 1-man business primarily using CRM under my Zoho1 subscription (I have NO other employees or users).  I wish to re-activate a prior user (my son, once more working with me) but when changing his status from inactive to active I receive this message:  "Regular User license limit reached
You can activate only up to a maximum of 4 user licenses. Please upgrade now to add more users."

Since I'm the only user and no one else I'm obviously under the "4 user license" limit.  What's wrong and how do I fix this?