I'm not able to enable Sync switch. Why?

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You must have reached your free plan limit. It will disable the sync and you will not be able to enable the sync till next month when your copy counter will be reset to zero. You can again enjoy the clipboard sync next month. Do not forget to enable the switch on the next month on all your devices to sync your clipboard.

If you want to enjoy unlimited sync between devices, you can check out the pro plans here.