You can achieve a lot in Denmark with English but it is much better to be able to work in Danish. Your customers may be mainly from your home country eg if you run a food shop importing products from your home region but you will still have to communicate with the Danish authorities either in English or in Danish.
You can learn Danish at Lær Dansk https://laerdansk.dk/en
You can also get help with Danish from the following free online courses.
Dansk Her og Nu https://danskherognu.dk/
You could try to get a Danish friend to speak with you in Danish by contacting Røde Kors, Dansk Flygtningshjælp or Venligborgerne