The reason:

When migrating from Jira Server/Data Center to Xray Cloud using the Jira Cloud Migration Assistant (JCMA), it's essential to ensure that all Xray issue types are identical in both instances before starting the migration, unless you're using the "Migrate all Data at Once" option.

If the Xray issue types are not aligned between Server/DC and Cloud, Xray cannot correctly map them, causing the migration to fail. Therefore, it's crucial to double-check the issue types in both instances before proceeding.

Important Notice: Matching Issue Type Names

The most common mismatch occurs with the Precondition/Pre-Condition issue type, as it is different on the Jira/Xray Cloud.  To avoid migration issues, rename it in Jira Data Center as follows:

• • Pre-Condition → Precondition

It's okay if you use different names for the Xray issue types. However, the names must match in both instances.

• For example:
   If an Xray issue type is called "Xray Type" in Jira Data Center, it must also be named "Xray Type" in Jira Cloud. 

How to check it:

In Jira Data Center/Server:

1. Go to Jira Admin > Issues > Issue Types.
2. Look for the following Xray issue types:
   • Test
   • Test Execution
   • Test Set
   • Test Plan
   • Precondition (ensure it is not listed as Pre-Condition)
   • Sub-Test Execution

In Jira Cloud:

1. Go to Jira Admin > Issues > Issue Types.
2. Search for "Xray" and confirm the presence of these issue types:
   • Test
   • Test Execution
   • Test Set
   • Test Plan
   • Precondition
   • Sub-Test Execution