What causes this issue?

Fewer depreciation areas are defined for the legacy data transfer as
there are depreciation areas on the asset. This is an error made during
Customizing.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Check the definition of the depreciation areas for the legacy data
transfer.

Error message extract from SAP system. Copyright SAP SE.