Adding a Page

https://musescore.org/en/handbook/measure-operations#append

In general, you don't add pages to a score in MuseScore - you just add more measures, fill them with music, and it goes to the next page automatically when it needs to. Just as you don't generally need to add pages to a document in Microsoft Word or any other word processor - just enter more text, and it automatically goes to another page as needed.

You can insert page breaks to cause it to go to the next page sooner than it otherwise would, but just as with most word processors, if there is no nothing else after the page break, then it won't have any effect. There needs to be something to display on the page in order for a page to appear.