Adding a Time Signature

What happens then when you try

Select a time signature, measure, note or rest, and click a time signature in a palette (double-click in versions prior to 3.4).

or

Drag and drop a time signature from a palette onto a space in a measure, or onto an existing time signature.

For me that adds a time signature.

Then please attach a sample score.
Explain where you wish to add a time signature, what procedure you follow, what you expect to occur, and what happens instead.

See:
https://musescore.org/en/node/289899