Error 491 in Google Play Store indicates that the memory is nearly full, and fails according to an update or installing apps.
If your device has a button to clear the memory, then press it. Otherwise simply close all open apps or reboot the device.
And if that does not help, then it may be that an app has to do with the fact that the error code has been spat. Some users report that it may be the Firefox app. But it can of course also be another Android app.
Otherwise, just as described below in our solutions, "Creating backup and reset to factory state".