According to your description above, you failed creating a migration end point in O365 side. What migration are you going to perform?
Could you please provide the complete error information you received when perform the migration and create the migration endpoint?
Have you tried using ExRCA tool to test if there is any connectivity issue?
Here is the official document introduces about the issue "The RPC server is unavailable" which occurs when you try to perform a cutover migration from an on-premises Exchange organization that uses Outlook Anywhere (also known as RPC over HTTP) to Exchange Online.
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