$200 Airline Credit "hack" with Delta Airlines

While I do travel a lot, most of my flights for the year were already booked prior to me getting the card in the spring.

I chose Delta Airlines bc they have a base in Atl and I'm going for Christmas anyway.

I don't need any luggage since it's a family home...and I typically just do super cheap airlines bc of it.

I tried the "hack" of buying an economy premium ticket one way refundable (only refunded my Delta account) $186- 1 month ago- credit never showed so I downgraded to main economy and now I can't "add upgrade" separately...

I thought maybe my mistake was buying it directly through Amex travel instead of Delta site...so I booked a BS flight directly to NY-2 weeks ago which I'm not going to in Jan-refundable of course but the "main econ" option just to see if it would work...still hasn't....

I will try to call Amex and see if it will work as it only shows the transaction "Delta airlines " and doesn't say what it's for.

Bottom line if that doesn't work...what do I do? How can I spend my $200 airline credit...I fly Delta on the 31st...literal last day of the year.

I know of course I can buy drinks and food on board...which I will do...but like how much food can I buy? 🤣 $200 seems like a lot

Any ideas? Tips? Tricks?

I've heard a separate upgrade could work...butttt bc I have basic econ ticket it says I can't do anything...maybe when I check un 24hrs before? I was hoping then I can ugrade? Most airlines I've noticed do have options upon check in even for basic econ