Unable to add a chord or notes within a bar without altering the other notes

I’m quite sure I’m not the best person to answer, but somehow that bar has become a 2/4 bar - you can see that with half note tied across the bar line so that’s what you’ll need to look for to address. You don’t appear to have your sign posts showing, at least not on the snippet so it’s hard why/how. I know there are some event posts here on that topic. Maybe look at this one?