You are using
AS COPY OF which is not supported in managed instance.
The link which you are using is for Azure SQL Database.
Notice in your URL that you have the parameter:
At the top of the page you have option to select the product/server that you used:
If you will select Managed Instance then the parameter in the URL will be changed to
If you select SQL Server (on premises) or Managed Instance then the document will not shoe you the option of
AS COPY OF
Moreover, in the UERL which you are using it is explicitly say :
Applies to: Single and pooled databases only.