I'm stuck at relationship 40? What do I do
First of all, I'd suggest reading what your character says whenever you try do gain more relationship exp
If yout skipped text, it's something along those lines: "I feel like my relationship isn't progressing, maybe I should talk with Yuna"
Which points you to the next step - chatting with her, start the night, choose living room, then give gift and "chat" to get a hint from Yuna on what to do next (a hint from me is to go running at night in the city specifically, after you get the info on what Yuna wants)
If all of the above is too much of a hassle, try searching comments or check out changelog on wiki, you'll find some clues or even straight up solution there
U have to go to the living room at night to talk (to talk to her you click the first option where you can chat with her) to Yuna where she tells you she wants to go running since she's starting to become chubby you than need to run at night where you meet the merchant (pizza girl) where she sells you the shoes for like 10000 I cant remember than you just go back home and gift the shoes to Yuna