As Dekans pointed out, this is an issue with Android 8.x. The delete file option was deliberately disabled due to issues with the API. I proved that the delete option works just fine by editing build.prop and tricking the system into thinking it was running Android 7.1.2 and API25.If you are using internal storage, then this is a bug, and nothing else. We don't delete on SD and OTG, because Android APIs are a mess for this. But internal memory should always work.
If not, open a bugreport.