Before migrating from Xray Server/DC to Cloud be sure you've completed the steps recommended at "Steps to a successful migration": 

• Have a valid License for Xray
• Enable Sub-Task in Jira Cloud
• Issue Types names are the same both on Server and Cloud

• Requirement ↔ Test Link must match both on Server and Cloud

• Review Issue Security applied to Xray Issues.
• Xray Custom Fields must be visible on Test Execution Issue Screens Scheme
• Run the pre-flight checks

Ready for a Checklist? Make Sure you do not forget any step and download our Xray Migration from Server to Cloud Checklist. 

Learn more at "Steps to a successful migration"at Migration Preparation.

Also make sure you are aware of the differences between Xray Server and Xray Cloud.

Be also sure, that if you select the Xray Cloud Standard you are aware of the storage limit and API rate Limit. If the storage and api rate limit, offered by Xray Standard is not enough, you might consider upgrading to Xray Enterprise.

If you want to migrate archived projects, you first need to unarchived them. 

Follow this article to understand how to check which projects are archived: [Xray DC] How to get all Archived Projects?