how can I give users acces to Synchronize Zoho & Goggle calendar

how can I give users acces to Synchronize Zoho & Goggle calendar

Hello,

I am the adminstrator and i have no problem to synchronize my zoho events with google calendar.

But it seems to be impossible for users.
I can't undestand why. User profile are ok for  Sync with Google Calendar (I have cheked).

But when users clic on Google apps, calendar, the system tell them " Insufficient Privileges to perform this operation. Contact your Administrator."

I have been seraching for a solution on guides, forum etc... but didn't find anything.
Could you please help me?
Thanks in advance,

Solène de Clermont-Tonnerre
sdeclermont@okavango-energy.com