Good day,
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: view=azuresqldb-current
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 view=azuresqldb-mi-current
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.