grandstaff

Welcome.
https://musescore.org/en/handbook/barlines#connect-barlines ?
Attach your score here if you have problems.

The easiest way to make a grand staff is to either enter an instrument like the piano that has a grand staff or press I to bring up the instruments dialog. You can then click on the words Staff 1 under the instrument name and click the Add Staff button (you probably don't want a linked staff so don't press Add Linked Staff). The barlines will automatically extend to both staves.

Try this : select a barline of first stave, right click----all similars elements of this stave-----all barlines of this stave become blue-----in the Inspector, you have in the middle BARLINES, with, under 3 numbers, the first is "1", put it on "2"