The fore-end line is inserted incorrectly.

Are you trying to add a grace note?
https://handbook.musescore.org/notation/expressive-markings/grace-notes

Are you trying to add a grace note?
https://handbook.musescore.org/notation/expressive-markings/grace-notes

It’s worth skimming through the Handbook first so that you have some idea of where to look for things like this. It has recently become much easier to use, with expandable subject headings on the left of the screen which you can scroll though quickly.

Score needed...

In the case of the first two examples, don't select the note you are slurring to. Just add the grace note, move it where you want and hit S. I believe a slur is automatically just to the next note unless specified by a range.