The IRS says my client must be in tax compliance, but what does that mean?

Tax compliance means the taxpayer has filed all of their outstanding tax returns (defined as the last 6 years) and are making their current tax payments.
Current tax payments are:
a. W-2 Employees: Withholding is sufficient, so taxes won\’t be due at the end of this year
b. Self-Employed: current year\’s estimated tax payments have been made
c. Business: current quarter\’s payroll tax deposits are made
You must also make sure that if a return will have a balance due, it is filed (and preferably assessed) prior to the filing of the offer. For example, if the taxpayer is in current tax compliance for year 2022 but his 2021 return will go in with a balance due, make sure that happens before the offer is filed as the taxpayer cannot incur a new balance.