Have you ever had a working attractmode? If so, what has changed?
Things to consider:
1. your attractmode .cfg file for the emulator is misconfigured
2. your MAME emulator mame.ini file doesn't have the correct "rompath"
3. your video driver isn't up-to-date or isn't correct
Look at the last_run.log for errors/details or run attract-console.exe to view errors in real time.