The cause:

This message comes up when at least one Pleo expense is from a year that's not yet created in Datev. Any expenses that don't match the selected year in Datev will fail to be exported.

The solution:

Please create the new financial year in Datev (see part 2.4 of their guide for specific steps). Once that's done, the export will work for newer expenses. Please note that, after creating the new financial year, no Pleo expenses can be exported from the previous years. We therefore recommend completely closing the currently active year first.