The subnets configured for the database availability group are not properly assigned to networks. Use the Get-DatabaseAvailabilityGroupNetwork ...

The subnets configured for the database availability group are not properly assigned to networks. Use the Get-DatabaseAvailabilityGroupNetwork cmdlet to view the configuration. You can then use Set-DatabaseAvailabilityGroup with the -DiscoverNetworks parameter to automatically assign the subnets.  The Set-DatabaseAvailabilityGroupNetwork cmdlet can be used to manually assign the subnets.