Per Kees-Jan at RPRO this is an error seen in older R4 builds. Client was on 721.163 when they saw this.
This was an old issue that was resolved. Updating to the current version should fix it.
You get this error, if there are no transactions in the batch when you enter Rpro and try to settle. So, it basically means the batch is empty… If you are already in Rpro and you’ve just settled a batch, and then try to settle batch (empty) again, it will give you a correct error (‘No unsettled batches’)