There are many possible reasons for having to file by mail rather than e-filing.
Since you mention that you are recently divorced, one very common possibility is that you and your former spouse are both trying to claim the same person(s) as dependents. The IRS will reject a return filed with a dependent SSN of a person who has already been claimed on another filed return.
Another very real possibility this early in the year is that you are attempting to file a return containing one or more forms that have not yet been “finalized” by the IRS. If this is the case, you should wait until the form becomes “available” before trying to e-file your return.
For more information about avoiding e-file rejections, please see Tax Tips for Avoiding E-file Rejections.