how to create a score

As MuseScore Studio is entirely free you can't have purchased it...
And the handbook describes how to create a score, see https://musescore.org/en/handbook/4

Hi there! I have just downloaded MuseScore 4 about two weeks ago.
There are many different ways to get information to help you get started.
1. work you way through the MuseScore Handbook - make sure that
the Handbook is the correct one for the MuseScore programme you have.
2. Wander through the many YouTube videos that people have produced.
Some of these are very good. When you find some that you like watch them
a couple of times. See how many helpful videos you cna find.
3. I have a little notebook (A5) and have written down all the Shortcuts
I find that are useful to me - and generally any helpful information.
I find it is easier to flip through my little Notebook than search through the
Online Handbook every time I want to remember something I have forgotten.
4. Begin small. Start with some test pages. Create a String Quartet or a
small ensemble - practise all the basic things like note entry, (etc.)
5. You can ask the MuseScore4 Community online just one question at a time
and people are very good at helping. I hope that, these ideas can give you a start!
Jeff (in Australia)

Short answer: File / New, or click the New Score button at the top left of the Scores page, or the New button at the bottom right of the Scores page.