Hmm... okay just to be entirely certain. From the mod page in knossos for the pack in question you went `Options` > `FSO Settings` and set the `FSO Build` to `FSO 21.0.0`? And it still says no executable?
If so, could you go into knossos' settings, expand the `Knossos` section, change your `prefered engine stability` to `nightlies`, check `show engine builds in mod list`, save the settings, and then look for the "FSO" mod and install the latest version of that. Then see if it works? It shouldn't be requiring a nightly build but... knossos is a bit fickle about stuff. I may need to manually edit the mod.json for the next release candidate. This should tell me if that's the case.