bless his heart, my dear husband just finished sanding, staining, and poyurethaning our hard wood foyer flooring for the first time.......all is dried and hard as a rock ALONG WITH SO MANY BUMPS AND BUBBLES!
What in the world can we do apart from sanding it all off and starting over?
Unfortunately you are going to have to sand it all down and start over. Sanding and finishing a hardwood floor is unfortunately harder than it seems, if you want a great looking finish. As to the polyurethane, there are a number of reasons you can get bubbles. First off, you need the correct brush for polyurethane, very different than a latex paint brush. Second, you have to be extremely careful never to shake polyurethane, you need to stir it very gently, then let the bubbles escape.
And when you put it on, you need to be very careful about not agitating it. It can be finicky about drying also, I never apply under high humidity conditions.
If you do try this again, I strongly recommending purchasing a book on floor finishing, read it first, then try again. Or hire a professional.