There are certain forms included in a tax return that the IRS does not support for electronic filing. An electronically filed return that includes one of these forms will reject with code R0000-225:
This submission contains a form or schedule that is not currently supported for electronic filing.
The list of forms that aren't support for e-file can vary each year, and sometimes a form that isn't supported in one year may be supported in the next year.
To diagnose, examine the return and look for an uncommon form, then check the IRS website and/or the form instructions to see if the IRS has published any information regarding the form's e-fileability.
Additionally, some forms may not be e-fileable during the early days of the filing season. For example, Form 4136 cannot be electronically filed before February 15. See here for more information.