It depends on when your OPT expires, for example, if it expires in a month and you have not filed for STEM extension, you would not be eligible. You must apply for OPT extension at least 90 days before your OPT expires.
As long as you can prove that you qualify for STEM extension and intend to apply for it 90 days before it expires. In addition, your employer must also be E-verified.
Please reach out for any additional question firstname.lastname@example.org