This problem is commonly asked by people around the world who are using Gmail. Such error pop-ups due to the unsupported browser which might be incompatible with Gmail or there must be some kind of interference with google operation by a browser extension. To resolve the issue the cache and cookies must be cleared by you from the browser.
Sometimes there might be some problem with your internet connectivity or with the Gmail services. And there may be one more reason for that due to the interference of privacy settings with Gmail.
Also, if the web browser is compatible with Gmail but still you are facing the issue then it might be the web browser is not updated. It may be still working on an older version which must be updated to resolve this problem. Also, check the extensions and add-ons that might be restricting the Gmail service.