Hi, @Anya23 Thanks for your question. I think the community may need more details before they can help troubleshoot.
It sounds like the 11-digit number likely refers to an object ID HubSpot is trying to use for an association, but it doesn’t seem to match an existing object. This could be due to an invalid or missing ID, or a mismatch in the “MyProperty” column you’re importing.
What object type are you working with? This will help narrow down the issue.