Why doesn't the child obey? What you need and do not need to tell your child to do your errands.
Why do I write so confidently about such things, it's just that I have four children, have experience in parenting and I like to read literature about parenting in order to replenish the wealth of knowledge and experience from expert people.
Once there was a time and I shouted at the children so that they would unquestioningly do what I asked them, but I had to spend a lot of time and effort, take a belt to intimidate, and then also redo the work for them myself. I decided to find out what I was doing wrong, I started looking for tips on the Internet, I found something for free, and something was paid in training courses.
So the child does not need to say: "do this or do that", especially if the assignment was for the first time and he does not have the necessary experience, it is better to speak in order of the actions that he must perform. For example: "My son, go to the kitchen and please bring a green mug that stands on the windowsill."