This can happen when no SDK has been selected for your project on which you are trying to launch the emulator.
Do alt+ctrl+shift+s to access your project structure and select the right sdk, apply, click ok and run it again
OR
Sometimes too it is due to AVG antivirus or windows defender that flags the adb.exe as a trojan horse(not true in this case) and blocks it access.
So just go to your windows defender scans and threats found, and allow the adb.exe to go through or whitelist it.
After that re-run the emulator it should work.