My h1b 2018 says received. My OPT expires Sep 30. Can I apply for a TN Visa on September 30. Does the 240 day rule apply to me?

You may be eligible for a change of status to TN, but it is not necessarily advisable. The 240-day rule does not appear to apply to you, because this is not an H-1B extension that has been filed. You may be able to take advantage of the H-1B cap gap extension. I encourage you to talk to your school's DSO or your employer's immigration attorney.

It is advisable to discuss your immigration dilemma with your employer's lawyer and also with your DSO.
The 240 rule is not applicable in your situation
Goodluck.
Ekaette Lawyers.

TN is sponsored by an employer. Won't your employer hire an attorney to assist with the process?