Non-paying tenant. What would you do?

[Miguel Gomez]

I am a newbie managing his own property. Rent was renewed for $2100 a month for a year. Tenant paid $20,000 in advance. $5200 remaining were divided in 11 months which came to $472 a month. Tenant failed to pay December 2020 on time, I forbid a late fee since she paid a big amount upfront, but came up with more excuses. 

Last Monday (28th) I wrote her that if I don't receive payment on or before Dec. 31st, a late fee will be applied. She answered another excuse and that it will not happen again. 

As of today, I have not received any payment.

What would you do?