Adding more notes on new lines

In the top menu, choose Add -> Measures -> Append Measures... and choose how many new measures you want. In case it's a localization thing, what we call "measures" in US English are "bars" in some other dialects. See https://musescore.org/en/handbook/3/measure-operations#append as well as the rest of the page for more information on adding measures to a score.

See above and add more measures. And maybe system breaks.

Unfortunately, music isn't English, and there are multiple meanings of "line" here. Each staff has five lines, for instance. A "grand staff" for piano has two staves, each of five lines. If your music is for piano plus voice, there is a third staff for that part. The three staves together make up a system.. A system is read left to right, then continues below in the next system.

So probably you are referring to a system - you want more measures after whatever you have. But you might also have meant, you want to add more staves to your current system, like to add a second voice part or a flute or whatever. Or maybe you're writing for guitar and you want a six-line staff instead of five.

Anyhow, to get more systems, simply add more measures (via the Add menu). The music automatically wraps around to create new systems as needed. Just as in a word processor, words wrap onto a new "line" automatically as soon as you have too many words for the current line.