Error Authenticating as Service Principal using PowerShell Script

I am trying to write some scripts to manage out PBI for an embed project. I am using the PS script below that I found online.  I am going this "Cannot bind argument to parameter 'Credential' because it is null" error.

I verified the Secret, Tenat ID and App ID are correct. I'm stumpped because there is clearly a value for the Client Secret so I am not sure why it is 'null'. 

I am new to PS so I'm hoping there is an obvious error. Any help is greatly appreciated.

Error:

Connect-PowerBIServiceAccount : Cannot bind argument to parameter 'Credential' because it is null.
At C:\Users\micha\OneDrive\Documents\_NBS_Solutions\_Customers\AgencyKPI\Powershell\Test Scripts\Report Distrobution Script.ps1:16 char:81
+ ...  -ServicePrincipal -TenantId $TenantId -Credential $PbiCredentialName
+                                                        ~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidData: (:) [Connect-PowerBIServiceAccount], ParameterBindingValidationException
    + FullyQualifiedErrorId : ParameterArgumentValidationErrorNullNotAllowed,Microsoft.PowerBI.Commands.Profile.ConnectPowerBIServiceAccount

Script:

#Variables 
$AppId = "*************"
$TenantId = "*************"
$ClientSecret = "*************"

#Create secure string & credential for application id and client secret
$PbiSecurePassword = ConvertTo-SecureString $ClientSecret -Force  -AsPlainText 
$PbiCredential = New-Object Management.Automation.PSCredential($AppId, $PbiSecurePassword)


#Connect to the Power BI service
Connect-PowerBIServiceAccount -ServicePrincipal -TenantId $TenantId -Credential $PbiCredentialName