Android phone users have been reporting a Google backup problem for months now, and Google has not responded to the problem. Select Android phones running Android Pie or Android 10 cannot create backups for their data on Google Drive.
Google Drive Backups

The issue is widespread, affecting many phone manufacturers, from OnePlus, Huawei to Samsung and even Motorola. Also, the problem has not occurred due to multiple accounts associated with a specific update or patch, or any other malfunction, or something similar. The cause of the issue is unknown, but given Google's tight silence regarding the matter, a workaround has been reported.

Many users have posted on Google Support Forums, their device manufacturer's forums, Reddit and other platforms to raise awareness about the issue. The report date has been over nine months, and Google has not acknowledged its awareness on the matter. To see if you are affected, go to Settings> System> Backup. The feature must be enabled, but some users may see the Backup Now button. If this button is unrecoverable, and all the data categories show the 'waiting to backup' sign, then you are one of the affected users and may not have even been aware of it. If the Backup Now button is accessible, you should not be worried.

As there is no word from Google, and no acknowledgment of an upcoming fix - a user workaround maybe your best 'temporary' bet. Such a workaround involves temporarily disabling all security authentication. The Reddit user claims that this problem arises due to the lock screen security method on the phone which now completes all backups from 2019 onwards. The temporary workaround involves disabling any type of authentication - be it a PIN code, pattern, fingerprint, or face unlock. Deactivating these security options should make the Backup Now button accessible once again. For some users, rebooting and/or clearing their Google Drive app data was additionally required.

However, this solution only works until its security options are disabled. Once you enable them again, Google Drive backup stops once again. Therefore, you must do this regularly, and make a backup if you are one of the affected users.

A second and more long-term solution requires ADB commands, and may also require the user to clear Google Drive app data. The entire process is explained in the XDA Developers Forum post, and it should activate the backup for users.