Having nothing to do with Covid or 2020, the IRS has always sent monthly letters asking for full payment on installment agreements; that's just the way their system works. IE., total tax due is $5,000, and you pay $500 a month:

1) Filing pay $500

2) next month pay $500

3) Get a letter says you owe $4,000 (plus interest).

This doesn't mean you must pay all $4,000 now. You must continue to pay $500 a month.