Which line is it failing on?

The Set-ADUser or the Remove-ADGroupMember?

Please use the code sample (101010) when posting code.

Try the following for troubleshooting purposes.

 $AccountList = (Get-ADUser -Filter "SAMAccountName -like '*$SAMAccountName'" -properties memberof, extensionattribute2)
    
 ForEach ($Account in $AccountList) {
 # Get Group List
        
     Try {
         $GroupList = $Account.memberOf
         Write-Host Clearing $Account.extensionattribute2 from $Account.SamAccountName 
         # Set-ADUser -Identity $Account -clear extensionattribute2
         ForEach($group in $groupList) {
             Write-Host Removing $Account.SamAccountName from $group 
             # Remove-ADGroupMember -Identity $group -Members $Account -Confirm:$False
         }
     } 
        
     Catch {
         $completionDetails = "There was an error removing the groups: $($error.Exception)"
         $completionDetails += "`nPlease investigate manually.`n"
     }
 }

 # You could also consider the following
    
 Remove-ADPrincipalGroupMembership -Identity $user -MemberOf (Get-ADPrincipalGroupMembership -Identity $user) -Confirm:$false
    
 # With a bit of error handling.
    
 Try {
     Get-ADUser -Identity $user
     #User exists
    
     $ADgroups = Get-ADPrincipalGroupMembership -Identity $user | Where {$_.Name -ne "Domain Users"} 
     # Domain users is most likely the users primary group which you can't remove.
        
     If ($ADgroups -ne $null) {
         Remove-ADPrincipalGroupMembership -Identity $user -MemberOf $ADgroups -Confirm:$false
     }
 }
    
 Catch{
     Write-Host "$user is not in AD"
 }

The first line of your script looks like it's the problem:

 $AccountList = (Get-ADUser -Filter "SAMAccountName -like '*$SAMAccountName'" | select SAMAccountName)

The variable $AccountList now holds a PSCustomObject (or an array of them). When you do this:

 $GroupList = (Get-ADUser $Account -Properties memberOf).memberOf

You're not using the samaccountname (which you assume to be in $Account), but a PSCustomObject. as the identity in the first position parameter of Get-ADUser.

Change that first line to:

 $AccountList = (Get-ADUser -Filter "SAMAccountName -like '*$SAMAccountName'" | Select-Object -Expand SAMAccountName)

Now you'll have a string (or an array of strings) that holds the user's samaccountname.

Also, there's no need for the parentheses on that line, either.