Hi Adam2467,
Welcome to Apple Support Communities. We understand that you're having an issue with your iPhone and iPad when trying to connect to your Google account. We'd like to help.
Just to confirm, when does this error message appear? Are you doing something specific on each device when it pops up? Let us know as that can help isolate this issue.
Sometimes this issue is related to the Mail settings within your Accounts. Do you have that email address in use on both devices? If you visit the Mail Settings Lookup, do the settings found here match with what is showing on your devices: Mail - Official Apple Support
If they don't, it would be best to reach out to Google to verify the data needed for your account to work on these devices.
Let us know how that goes.
Take care!