Please make sure that you have the latest versions of Google Play Store and Google Play Games (Huawei App Gallery/Amazon Appstore/Samsung Galaxy Store). If updating these applications does not help, please contact us and we will suggest possible solutions of this issue.
On iOS, you need to make sure that "Game Center" is active. In the latest versions of the operating system on Apple devices, Game Center does not exist as a separate app, but as a section in the list of device settings. If GC is active, you may need to re-login in this section.