"Couldn't upload. Try again." error on Google Play Console

Google Chrome

Use a new Incognito window to upload the .aab or .apk (as mentioned in the comments).

Alternatively, clear the website data:

1. going to: developer console (F12) > Application > Clear storage > Clear site data
2. press F5 to refresh the page
3. uploading the .apk/.aab again

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Safari

Open Preferences > Privacy > Manage Website Data > Remove All > Done

This problem comes out due to multiple gmail logins on same browser, if the developer account is not the default logged in account. You can solve this by signing out all gmail accounts and login in the specific gmail account.