A rabbit coughing is very unusual and usually means the rabbit is experiencing significant respiratory difficulty. Which usually means this is likely an emergency and the rabbit may need immediate intervention by a knowledgeable rabbit vet, with oxygen and IV antibiotics. Your rabbit may have even developed pneumonia, which is also an immediate emergency, which requires immediate respiratory aid and IV antibiotics or the rabbit isn't likely to survive.
https://wabbitwiki.com/wiki/Pneumonia
(LINK CONTAINS GRAPHIC MEDICAL RELATED PHOTOS)
https://www.medirabbit.com/EN/Respiratory/Illness/Pneumonia/Pneumon_en.htm
Call the vet today and tell them it's an emergency and your rabbit is having difficulty breathing. See if they will let you come today. Or if there are any other knowledgeable rabbit vets where you live, see if one of them will see your rabbit today.
If you don't have any other options for a good rabbit vet where you live, and your current vet can't fit you in today and you have to wait until tomorrow, you could try steam from a bowl or bathroom, to see if that helps. Just be careful though, if using hot water around your rabbit.
And do not get your rabbit wet at all.
There's also a suggestion in this link, of adding Vicks to the hot water, and the steam from that to help open up breathing passages. But it's not something I've tried. And if in doubt, phone your vet and ask for suggestions on how to help your rabbit make it until your appointment tomorrow. They may suggest using something like a bulb syringe like used for babies, to help remove some of the mucous from the nasal opening (done with care and under vet advise), to help alleviate the breathing a little. But if your rabbit has developed pneumonia, this may not help much.
https://rabbitwelfare.co.uk/rabbit-snuffles/