Hello,
I've installed this plugin and clicked "Settings" at Installed Plugins menu.
I have a message "Sorry, you are not allowed to access this page."
I definitely logged-in as admin and I can not see "Forums" menu at the side menu.
Please navigate to Dashboard > Users > Your Profile admin page, scroll down and find wpForo Usergroup dropdown, make sure it's set admin for you.
The dropdown list is empty.
I've attached the screenshot.
That's too bad. It means the installation was incomplete. You should remove wpForo totally and try to install it again. I hope you don't have important data in forum yet. Please navigate to Dashboard > Plugins admin page and click on Uninstall button of wpForo plugin, confirm it. Then delete wpForo and install it again.
I tried three times and it still didn't work.