Name of the deployment preview error
Data instances are not supported in Validation deployments
Sample error message
Data instances are not supported when executing a 'validate only' deployment. Therefore, Instance [MyProductName] will be omitted from the deployment plan
When will I see this deployment preview error?
This error occurs when you select a data instance, i.e., records like Products or CPQ Pricing Rules, and you run a validation deployment (a.k.a check-only deployment) to your Salesforce org.
Check-only deployments roll back the changes after they are done, allowing you to later deploy the same metadata without running tests.
Check-only deployments are only supported for metadata. Salesforce does not provide a mechanism to roll back data insert/update/deletion changes.
How does this affect my deployment?
The preview error is letting you know that only metadata items (if any) in your deployment plan will be part of the check-only deployment, and object records will simply be omitted.
When you do a "real" deployment, the object records will be included as expected.
How can I fix this error?
This is just a warning so that you understand the implications of a check-only deployment that includes data records. There's nothing to fix.
You can learn more about deployment preview errors here.