issues with installing update

issues with installing update

i have a Macbook Air, which was running on OS X Yosemite 10.10.5.

today i tried to upgrade to High Sierra, as i was having issues with Safari.

at the AppStore, it asks me to enter the Apple ID password to proceed. there were 2 users logged in on the laptop, I've logged out of the guest and tried to update via the admin. when i type the password in it says "incorrect address", that either the ID or password are wrong. i changed the password and tried again. it continues to say that.

it asks to enter the password and then redirects to Billing Info, where it says that there is a request error.

the laptop is a post-2009 version.

why won't it update? how can i update it?