I'm not able to login my FaceTime on my MacBook Air, I'm using it on my iPhone but still not able to use it on my MacBook. What should I do?

Thanks for the additional information. I would try a restart first, then:

• Choose Apple () menu > System Preferences, then click Date & Time and select “Set date and time automatically”. Click Time Zone and make sure that the settings are correct. 
• Make sure that you've installed the latest software updates.

The other helpful tip might be to make sure your Wi-Fi isn't strained as that could affect the connection.

Other tips are included in this support article from Apple: https://support.apple.com/en-us/HT203585

I hope that does the trick!

Does it not accept your AppleID and password? Or is there another issue?