[PS] C:\Program Files\Microsoft\Exchange Server\V14\Scripts>.\new-TestCasConnectivityUser.ps1
Please enter a temporary secure password for creating test users. For security purposes, the password will be changed r
egularly and automatically by the system.
Enter password: **********************************
Create test user on: Server1.test.local
Click CTRL+Break to quit or click Enter to continue.:
CreateTestUser : Mailbox could not be created. Verify that OU ( Users ) exists and that password meets complexity requi
rements.
At C:\Program Files\Microsoft\Exchange Server\V14\Scripts\new-TestCasConnectivityUser.ps1:267 char:31
+       $result = CreateTestUser <<<<  $exchangeServer $mailboxServer $securePassword $OrganizationalUnit $UMDialPlan $
UMExtension $Prompt
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException,CreateTestUser

This error is because you have several OU’s named “Users”. Edit the scrip to use a spesific OU, i.e “CN=Users,DC=test,DC=local”