The two databases
Your state and the IRS check different lists
State Secretaries of State check name uniqueness against active filings in their state only. The IRS checks against a nationwide database that includes dissolved entities, disregarded entities, and entities filed in other states. A name that's state-unique can still trip code 101 at the IRS.
Three patterns
What usually triggers 101
Most 101 rejections fall into three buckets: a previously dissolved entity still has an open IRS record, an active out-of-state entity has a substantially similar name, or punctuation/designator differences (LLC vs L.L.C.) collapse to the IRS's matching threshold.
The fix
Mail or fax SS-4 — agent review can clear it
Online retry with the same name will fail again. File SS-4 by mail or fax; an IRS agent reviews and either accepts the name with documentation or returns a notice requiring you to amend the name with the state first.
Related codes