Recommendations for a first card as a grad student. Already denied pre-approval from Discover and need some guidance.

Current cards: none

FICO Score: Sallie Mae FICO report: 693. Discover soft pull for pre approval: 688

Oldest account age: n/a

Chase 5/24 status: ??

Income: $30,000

Average monthly spend and categories:

dining $200

groceries: $400

gas: $100

travel: $100

other: $100

Open to Business Cards: No

Building credit and cashback.

Discover IT was the card I requested pre-approval for. The American Express Blue Cash Preferred Card caught my eye with its rewards, but I doubt it's a good option for my current situation.

I'm okay with category spending.

Additional info: I'm a first year PhD right out of undergrad earning a stipend of $30k annually and getting tuition covered by the university. My housing is $1k monthly. I have student loans from my undergrad, which I pay some interest toward monthly and will be deffered until I graduate with my PhD. In the rejection letter Discover said that I don't have enough income to be approved for any of their cards, which honestly just doesn't make sense to me, since they have student cards that are approved for graduate students. Is there anything I should do to make this easier? Or any other cards I could apply for? I don't plan on accumulating debt (I have enough of that as it is from student loans) but want one for cash back and building credit. Any help is greatly appreciated