I have a tax bill and I never filed these tax returns.
What are these?  They are substitutes for returns prepared by the IRS.
Many times people forget to file their tax returns and the IRS finally picks up on the fact that there are no matching returns for a social security number.   What happens next is that the IRS has the authority to file for you. This is what they do. They get all the income information they need that was submitted to them by third party sources such as  banks, employers, 1099 sources, and then the IRS prepares the tax return for you. They prepare any back years they have information on. Then, the IRS gives you the standard deductions,  prepares the return and serves you notice that this is/are the returns they have prepared for you.  You can reply and change the amount of the back taxes, appeal or just do nothing.
If you fail to reply, the IRS will  assess the tax liability and in a prescribed period of time, send you out notices and bills to  pay the back taxes in full. Should you fail to respond to their last notice they will take enforcement action which will include the federal tax lien and start sending out levy notices to your employer. They could also  send notices to your bank and possibly seize all funds in the account.
What usually is the cause is the fact that the taxpayer did not respond and in over 50% of the time, the taxpayer  moved and never received their tax bills or back tax letters. They find out about the assessment only after the IRS has levied or put a lien on their bank account account or levied their wages.
In situations like this, Settlement Offers, Offers in Compromise or Doubt as to Liability may be prepared and new tax returns sent into the Internal Revenue Service.
Be careful and pay attention to any notice sent out by the IRS.