There might be additional reasons such as the OP was traveling to the US on this connecting flight where he was selected for secondary screening by the US government where online check-in is prohibited in this case, etc...

Or there might be another passenger under the same name on the same flight which would also block the online check-in process. Such as if the OP was named under a common name like " Steve Miller ", etc...

Without knowing all the details ( where to, where from, flight details, etc... ) we can't comment on the situation.