As we per our understanding from our clients who used Roku devices.
Recently, Roku has updated its system & software for some models. Due to this update, the developer mode can't be accessible for some Roku models.
Please try to reset your Roku device to factory default settings. Then try again to install the app again.
If you still have the same issue, please log in to your online account and submit a support ticket and we will help you with an alternative solution.