Diagnosis
After a factory reset, the router reverts to its *default* credentials, not a blank one. You're probably entering the wrong default password or the browser has cached an old login page.
How to Fix It
The default username is admin. For the password, look at the sticker on the bottom of the router itself. There should be a default password printed there (it's often blank/empty, but some versions use a unique code). Try leaving the password field blank first. If that fails, use the exact code from the sticker.
If That Didn't Work
Clear your browser's cache and cookies, or try a different browser entirely (like Firefox if you use Chrome). Also, make sure you're accessing the setup page at the correct address: http://dlinkrouter.local or http://192.168.0.1.
Heads Up
If you previously set up a custom admin password and wrote it down, that's the one you need *after* setup. The factory reset wipes that and goes back to the sticker password.