I am wondering if a person with adrenal insufficiency)Addison’s disease) is eligible for the Canada disability tax credit?

The test for the DTC is not necessarily dependent on having a specific diagnosis. Rather, it's a functional test. In theory, someone without any diagnosis at all could satisfy the test, if their symptoms were severe enough.

You can read about the test, and other eligibility requirements on their website at: https://www.canada.ca/en/revenue-agency/services/tax/individuals/segments/tax-credits-deductions-persons-disabilities/disability-tax-credit.html.