I tried to post this a comment but no matter what I did it would not work. I'd click on the post/submit button and nothing would happen.
Posting the same text (simply copied from the comment) as an answer (even though it's not an answer) works.
Here's the text:
IanXue-MSFT has given you a way to fix your problem -- by piping the expected object type to the Remove-ADUser. However, you should also understand that you can use Select-Object -- but you have to provide a property in the resulting object named "Identity". You can check the help for the Remove-ADUser to see that you can pipe that property name and provide a sAMAccount as its value. To do that you'd use a calculated property:
Select-Object @{name='Identity';expression={$_.sAMAccountName}}
As IanXue-MSFT pointed out, you don't need the Select-Object in this case. Also, note that the Remove-ADUser will, by default, prompt you for permission before it removes the account. See the Notes section of cmdlet's help:
"By default, this cmdlet prompts for confirmation as it is defined with High impact and the default value of the $ConfirmPreference variable is High. To bypass prompting for confirmation before removal, you can specify -Confirm:$False when using this cmdlet."
