The IRS uses Notice CP15 to tell you they have asserted a "frivolous return" penalty against you. This is a very dangerous notice not only because the penalty has been unilaterally assessed but now you are subject to greater scrutiny because the IRS has labeled your return as "frivolous".
Notice CP15B is used to impose Employer Business taxes on you personally as a Responsible Person for the Trust Fund Recovery Penalty.
Response to CP15
It is possible to appeal this determination, even with a Collection Due Process Appeal but the IRS is not likely to reverse their determination without some very compelling evidence.
The traditional method (& the only sure way to have your case heard) is to pay the penalty (or a portion thereof) and then file a Form 843, Claim for Refund. If the IRS fails to act or denies your claim, you can sue them in US District Court.
But, it will take a fair amount of work to reverse this penalty. There are traps and tricks for the unwary along the way.
Because the assessment is unilateral, the exact reason for the penalty won't be revealed. Therefore, you may have to make a Freedom of Information Act request (FOIA). See the FOIA Guidelines & Requirements.