There is a known issue in Importacular version 4.9.0.1 that is resulting in Benefit mappings not being imported to Gift Batches. This issue has been resolved for the next release (due to be live for all users in Q3 of 2021).
In the meantime, there are two workarounds:
1) Rather than import to batch, import directly (see this section of our User Guide to learn how).
2) If you must import to batch, do so and then add your benefit(s) separately (either directly in the batch, via a Global Change or via a Raiser's Edge Import).