Install themes does not recognise admin password

I’m really new at WordPress, but have played around on an installation offline. This is a problem with the online version. It was not there before, but is there now. It goes like this:
I find a theme via the search in the appearance section of the dashboard, I go install, it requires a password, I put in my admin password, it says not recognised or words to that effect. I’ve tried clearing the cache logging out and back in again. It does not help. My admin password will log me in, but not allow me to install themes from the dashboard.
Not urgent but a solution would be nice. Thanks for taking the time to read this post.