Every year my w-2 is check marked as retirement plan on box 13, but this time it is blank. I'm still under the pension plan. Nothing has changed. Should I leave it blank?

You should ask your employer why they left it blank this time if it has always been checked for past years.

It seems like it's an omission from your employer.

You may want to reach out to the employer to point this out. And they might issue a corrected W-2, which you'll use instead of the one you have.