MS4.2.3 cant add a pitched clef when changing into a pitched instrument from initial percussion staff

First, never use octave clefs except for instruments where these are potentially expected (piccolo, guitar, bass, that's about it). Ottava lines are the way to go for all other instruments if you need to avoid ledger lines.

But for the record, since other clef changes (like treble to bass for a one-staffsynth part) certainly are a valid thing:

There is indeed an issue where if the initial instrument is unpiched, you won't be able to insert manual clef changes even if adding isntrument changes later. So the solution is to change the initial instrument. In your score, use the Instrument panel to change the instrument to something pitched, then if necessary do an instrument change to unpitch in the first bar. At least, that works in scores created within MuseScore. It seems this one was imported via MusicXML, which sets things up differently based on the info in the file itself and is probably the source of any staff configuration issues. Might be best to add a new instrument and then copy the content into it. When I tried that in your score, it worked fine. But agaiun, I definitely don't recommend it. Keyboard players won't enjoy the mental arithmetic. For that matter, they won't enjoy being asked to double on snare drum. Unless of course you've already worked that out with the player you are envisioning playing this part.