That Microsoft account doesn’t exist. Enter a different account or get a new one
In order to add apps to SharePoint you need to use a Microsoft account. This account is not the same as your work or school account and would therefor usually give you this error when trying to add the app.
Simply sign in to a Microsoft account or create a new one and proceed with the installation.
Note: this account does not require any permissions to your SharePoint site and does not need to be associated in any way with your user licenses.