"can not sign out of my apple id on my Mac: my macbook air is using OS Catalina, since i'm selling it to my friend now and i need sign out my apple id so he can use his own apple id. i' been trying to sign out but still don't works. just processing and processing.. my macbook is connected to the internet all the time."
Two Things:
A. Verify this Device:
Verify that this device is logged in to your account, by using this link: Check your Apple ID Device List to See where you're Signed in - Apple Support. If not, then it is not even logged into it.
B. See if you can just erase the device:
Go Here: What to do Before you Sell, Give Away, or Trade in your iPhone, iPad, or iPod touch - Apple Support