I was trying to install the first app to my store and came across this issue.
When I open apps.shopify.com from my admin page, eventually it will be not logged in and when that happens, I'm unable to proceed with the app installation.
I can reproduce that by directly opening apps.shopify.com and trying to install an app, after I fill my store URL, it will raise an error due to CORS restrictions.
Open a tab with your account logged in, then open a second page, go to your account too, click on Apps » Visit the Shopify App store
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Hi @Helloweb , thank you for the response.
Eventually I was able to install the app, but decided to post here since it's not an expected behaviour and took me a while to figure out a workaround for this