Get function is finding deleted users - UserIds are not accepted by Invoke-MgDismissRiskyUser
Perhaps due to some change since this code was written, or an oversight because it was never tested in a live environment, the function in "Get-AzureADIPRiskyUser.ps1" gets users who have been deleted as well as current users.
The result being that if you then take that output over to "Invoke-AzureADIPDismissRiskyUser.ps1" (or even send that output directly to Invoke-MgDismissRiskyUser) there are user ids included that the command deems "invalid".
The "Get-AzureADIPRiskyUser.ps1" code can be modified to include a filter to exclude these deleted users on line 116:
$filterRiskLevel = "(RiskLevel eq '$RiskLevel' and IsDeleted eq false)" -f $RiskLevel
This is probably not the most elegant solution. But it worked for me.