Unable to install or update apps using App Store as "Media & Purchases" section is greyed out

Hi, I've tried so many ways to be able to sign-in Media & Purchases (App Store) through system preferences with the right username (email address) as well as the right password. But it still being greyed out which makes me unable to install or update apps using App Store

Kindly please help me to resolve this issue. Thank you

Cheers,

TSRS

[Edited by Moderator]

Original Title: Media & Purchases (greyed out)