With an /oauth2/authorize URL like that, the "Please sign in with a work account." message indicates that the app key in the client_id URL parameter is for an app that uses some team scope(s), but the account you're signing in to is not on a team.
When using any team scopes, the app would be connected to the entire Dropbox team, in order to use functionality specific to Dropbox teams. Accordingly, only Dropbox team admins can authorize such apps, and the Dropbox web site would prompt you to sign in to a team account accordingly.
Upgrading to a team account would resolve that, but that would only be necessary if you do need access to team functionality. If you don't need any team functionality, you should instead disable any team scopes on the app. You can manage the app's scopes from the "Permissions" tab of the app's page on the App Console while signed in to the account that owns the app.
You can see which scope is required for each endpoint in the API documentation. You should enable only the scopes needed for the functionality in your app.
