Eunice here from Shopify, thanks for posting, and welcome to the Community!
From your message, it sounds like you're having issues with logging into the Shopify App Store and installing an app. I'm very sorry to hear this, let's see if we can fix this by clearing your cache and cookies. This guide shows you how.
Our browsers hold onto old information and can cause issues like this to arise. It's recommended to clear your cache and cookies on a regular basis.
If you're still not able to install an app from the Shopify App Store after clearing your cache and cookies, please visit the Help Center and log in to your account to create a support request. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback.
Hope this helps!