Azure Active Directory cannot provision more users to Salesforce.com on your behalf, because your user license limit has ...

Azure Active Directory cannot provision more users to Salesforce.com on your behalf, because your user license limit has been reached.  For Salesforce.com developer environments, you are limited to just two regular users, although you can have an unlimited number of Chatter Free users.  
 
You have two options.  One option is to purchase additional user licenses from Salesforce.com.  The other option is to change the access you have granted to your users, granting only Chatter Free access to any users in excess of your license limits.