Adjusting layout of score

Reduce the 'Space' value, in the Page setting dialog

Even though music is indeed more complex than text, it's actually still the case that the bulk of what you ordinarily need to do is done with exactly the same controls one uses for text. And in particular, it was definitely true for the original question you asked about fitting more on a line - that much is exactly analogous. But it goes beyond that example - quite a bit more about music formatting derives directly from concepts established with text. So, far from being unhelpful, it's incredibly useful to know that your accumulated knowledge about how text works can be applied to music as well! And indeed, often with the same terminology. So don't assume your existing text knowledge will be worthless with music - it won't be, I promise!

In your score, the extra white space I can see is mostly about the margins above and below the lyrics - would you agree? So, just as a word processor give you control over the margin above/below a paragraph, MsueScore gives you control over the margin above/below lyrics. See Format / Style / Lyrics. For music like this where the lyrics actually apply equally to both staves, you probably want equal margins (the larger lower margin makes more sense in music where the lyrics only apply to the staff above and so you need additional separation between them and the unrelated staff below.

Setting lyrics margins above/below works pretty much exactly like text, but some of the other things going on here are more music-specific, so then you need to get into other techniques. For instance, maybe you could reclaim some space by shortening the stems on some of the alto notes that extend below the staff, or the tenor notes that extend above. Or maybe move a slur closer to the notes or flatten it. With text, that's not usually an option - you can choose the just shorten the descender on a "j" to save space. But you can with stems.

Aside from that, I go back to concepts from word processors - page margins, header footers, etc - that are carried over with some slight modifications. Right now you have a 15mm bottom margin. That could certainly be reduced. You can also safely reduce the "music bottom margin" in Format / Style / Page. The default of 7 sp is designed to allow for more markings below the staff without encoaching on the page margin or creating a ragged look from page to page. But in closed score vocal music like this, most markings are between staves, so you don't need so much extra space, and if the goal is to have one page only, ragged bottom margins won't be a concern. So you can actually lower that value a lot. You can also reduce the size of the title frame.

You also have disabled vertical justification of staves, which is quite counterproductive - it prevents MuseScore from distributing space between staves more intelligently/professionally. Set that back the way it was, but also reduce the min system distance in that section of the dialog as you did the other.

Do these things and you'll see MuseScore intelligently fill out the page for you, but gives you extra room to work with, so you can easily add two more lyrics verses, and MuseScore re-space the staves automatically.