When registering for the SEDR Cloud portal, the serial number needs to be a SEDR serial number. You cannot register for the SEDR cloud portal with an ATP or SEP license.
You can verify that you have purchased the SEDR product by reviewing the .slf license file in a text editor and looking for the following in the name fields:
<product_name>Symantec Endpoint Detection and Response</product_name>
Please review this Community post regarding the end of support for SEDR Cloud/EDRC: