The Essential plan from Nationwide Mutual Insurance is for travelers planning trips on a budget who don't want to sacrifice coverage. Coverages include Trip Cancellation and Interruption, Medical, Emergency Medical Evacuation, and Assistance Services.
To learn more, see the Nationwide Essential trip insurance product page.
According to your policy, "Pre-Existing Condition means an illness, disease, or other condition during the sixty (60) day period immediately prior to the Effective Date for which You, a Traveling Companion, a Family Member booked to travel with You: 1) exhibited symptoms that would have caused one to seek care or treatment; or 2) received or received a recommendation for a test, examination, or medical treatment; or 3) took or received a prescription for drugs or medicine. Item (3) of this definition does not apply to a condition that is treated or controlled solely through the taking of prescription drugs or medicine and remains treated or controlled without any adjustment or change in the required prescription throughout the sixty (60) day period before the Effective Date"
Unfortunately, you were not eligible for the waiver since you did not purchase the policy within 10 days of your Initial Trip Payment:
"The Pre-Existing Conditions exclusion is waived if You (a) enroll in this Certificate at the time You pay the deposit required for Your Trip (or within ten (10) days of the initial deposit); (b) purchase this Certificate for the full cost of Your Trip; and (c) are medically able to travel on the Effective Date.
Of course, it is never pleasant to hear that your claim was denied. Nationwide confirmed that you were sent the letter so you may want to check your spam/junk e-mail folder for the information.