1. Press the Windows keys on your keyboard
2. Type "Run"
3. Click the Run Desktop app that appears in the search
4. Type "appwiz.cpl"
5. Look down the list at the bottom. Is "World of Warcraft Classic" listed? If no it has not installed correct, so install it again. Try installing as an administrator see if that helps.
It also helps if you give some information about your operating system, hardware specs
