How will I know if I’m able to submit biometrics through the UKVI app?

I submitted my naturalisation application and was directed to the UKVCAS website. I received an email from UKVCAS that if I’m eligible to book an appointment through the UKVI app that I won’t be able to book an appointment immediately. However, when I log in to the UKVCAS website it says I must attend an appointment and it gives me the option to book an appointment. Does this mean I won’t get an invitation to use the app? I’m not sure why this would be the case as I submitted my biometrics earlier this year, so I’d think reusing them wouldn’t be an issue.

you should've received an email asking you to wait until UKVI processes eligibility for app usage instead of booking an appointment.