Sure thing :D then remake it the same.

Either make it the same, or ask if they’d like it sweeter (and if they say yes explain that the recipe calls for however many i gave but i’ll put in ___, that way they can order it that way again or at least hopefully not give their next barista the same complaint)

I'd tell them I'm not familiar with their regular order and ask them to bear with me while I try to get them what they want, then have them repeat the order to check the sticker/ring-up was actually what they asked for, then have another partner remake it for them if possible.

If it's still wrong and they still can't tell you what they want to change in the build (sweeter? different kind of milk? an extra shot of espresso?) then apologize for not being able to help more and offer to make them something else, or a refund.

This happened to me. I wish the dude came into the store because he was holding up our drive thru line. I ended up making four venti iced white mochas on the spot because each time we'd give him his new drink he mumbled something like, "its too sweet" "it's too watery" "it's too bitter" DO YOU HAVE NOTHING BETTER TO DO WITH YOUR TIME!? And, in the end, I made him a regular venti iced white mocha with light ice and he liked it. (It makes me wonder who made his drink last time.)

I would probably say “I’d be more than happy to remake your drink if you can let me know what you’d like changed, or I can make you something else or give you a refund if you’d prefer”