Notes with different lengths are incompatible with each other

I'm planning to make a score on Tchaikovsky's "Souvenir de Florence" but I can't make the first note.

Watch this video to understand. https://www.youtube.com/watch?v=BM7EQz-vl4s
Anyone how responds will be thanked a lot. (I am not great with Musescore)

Sincerely,
Jadon L