Error during synchronisation. Zoho CRM could not push Contacts updates: Could not communicate with ZohoCRM: [4421]: You crossed your license limit

Error during synchronisation. Zoho CRM could not push Contacts updates: Could not communicate with ZohoCRM: [4421]: You crossed your license limit

I'm on the free account. My syncing software is giving me this error from zoho crm. Why? I read nothing about restrictions of api calls to the free account. Thanks