Every time I'm gonna change the theme that I see as I browse through the site in the "Site Appearance" tab in Settings, it does not work, because every time I click "Save", a message appears saying "Theme does not exist", and nothing happens, how do I solve this?

Try changing the "Overwrite Site theme with chosen theme" dropdown

What value did you have which wasn't letting you save?

Try reset your website and upload it with guidelines below:

Reset site
Try to Upload theme file after some time
Make sure your item is wordpress version.

Regards,
Rachel Gomez