If you cannot enter coupon codes at the end of the campaign creation process and are instead asked to enter the number of "Required product testers", it is because you selected the option "Limited offer" in step 2. Only select this option if you intend to create a campaign with a time limit in conjunction with a price reduction directly on Amazon.
So go back to step 2 and deselect the option. The coupon code field will automatically reappear in step 7.
▶️ Also interesting: You can find out everything about campaigns with a time limit in conjunction with an Amazon price reduction in the help center.