A CP16 Notice is good news and bad news. The IRS believes you made a mistake on your return and are entitled to a refund. But you owe taxes for a different year, so the IRS is keeping your refund to pay that debt.
If you received a CP45 Notice from the IRS, it means that they can’t apply your refund to your estimated taxes as you requested. Instead, they will apply your refund to a previous tax debt.