Can't set up Adobe Stock contribution acount

A contributor account is not connected to an Adobe subscription. Sometimes people use the same Adobe ID. It would not be a good idea to use a school or employer ID for Adobe Stock contributor, because if you leave the school or the job you then lose all your images and future earnings!

I would not doing a Contributor account with an Adobe id that is not under my control. There are a few disadvantages*, but my advise would be to create a new Adobe id with an e-mail under your personal control. You do not need to have a plan attached to this id to start being a contributor.

If your subscription is an individual subscription (as a teachers/student plan) you should not have a problem to create a contributor account. So I suppose you have an Enterprise plan, as provided by your school. Those plans are not eligible for stock contributions.

*you can't submit to Adobe stock via Lightroom Classic. This is in your case a minor issue. It is easy to work around and only implies exporting first and then submitting that exported file.