Unable to Install phpBB

I'm trying to install phpBB in a sub-folder, not in the root folder but it won't install.

This is the error message I receive.

phpBB Installation
Error Installation failed
You cannot install the application because there is another application installed in the root folder of your site.

Can anyone please help me with this?

Thank you.

Isn't the message clear enough? You have another application installed in the root directory, which is WordPress in your case. You have to remove that from the Applications section of the AlterVista control panel.