US Immigration from Congo DR

Hello guys,

I have a question about when you file for N-400. My wife and I got married on 01/12/2018. So, it will be 3 years on January 12, 2021. USCIS advises to file N-400 within 90 days before the third anniversary (in the case of a marriage to a US citizen that I am).

This is my question: what happens if you fail to file N-400 within 90 prior to the third anniversary? To be clear are we supposed to necessarily file the N-400 within 90 days prior to the 3rd wedding anniversary or we can do it sometime next year like in March or April 2021? What does the law really say about the time to file? Please assist.

Notes: her conditional resident status has been extended for 18 months after applying for I-797 as the biometric has not yet been done (certainly because of COVID-19). Does it mean that she is good to proceed with the N-400?