reproducing an old score ... help!

I've been trying to reproduce a score from a very old book and have managed to get to bar four before getting stuck!

I'm no officianado so please excuse my lack of musical jargon. In the fourth bar (please see attached) you will notice that there is a minum followed by a crotchet, the music being in 2/4 time, musescore won't allow me to enter these notes. Any suggestions on commands I might have missed or work arounds I might not be aware of.

I tried searching previous discussions for this but I couldn't even think of what the search terms might be!