First off, what directory is your forum in?
If that's what it is, make sure you upload the template to the 'templates' file under your root phpbb directory.
And delete the template file if you uploaded it elsewhere.
I think you may have deleted the default subSilver template.
That's your problem.
Upload the subSilver template.
Then, DO NOT DELETE subSilver.
Add your template file in the templates folder.
Go into your admin panel.
Under Styles, Click Add.
Install the template you added.
NOW, set that as your default template.