It seems that the script sees the checkbox for "All guest and external users" in the exclusion are of the Users and Groups management as a2c3906b-07de-420d-a036-f005a85806e7 and fails trying to look it up as a user.
VERBOSE: GET https://graph.microsoft.com/v1.0/users/a2c3906b-07de-420d-a036-f005a85806e7 with 0-byte payload
Invoke-RestMethod : The remote server returned an error: (404) Not Found.
At C:\Program Files\WindowsPowerShell\Modules\DCToolbox\1.0.18\DCToolbox.psm1:895 char:29
+ ... ryRequest = Invoke-RestMethod -Headers $HeaderParams -Uri $GraphUri - ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
+ FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand
It seems that the script sees the checkbox for "All guest and external users" in the exclusion are of the Users and Groups management as a2c3906b-07de-420d-a036-f005a85806e7 and fails trying to look it up as a user.