Help with formatting system breaks

With the long instrument names there is only room for three measures on that first page. You have a number of options to choose from including,

• rename the staves to something shorter
• reduce the margin sizes
• reduce the scaling to make everything smaller
• accept that having 4 measures per page doesn't work for this score and leave Musescore to decide where to break pages.

My preference would be the last one as Musescore generally does a good job of page layout. But for the others, the first one can be achieved from the stave properties accessed from the right click menu, the second and third can be done from Format>Page settings.

See also https://musescore.org/en/handbook/4/page-layout-concepts
and https://musescore.org/en/handbook/4/pages-and-vertical-spacing

An alternative suggestion - force the instrument long names onto two lines with the Enter key: