Coupons should be ideally part of the Promotions in Drupal Commerce 2. But even if you have not added any promotion, a default coupon redemption field can be enabled, leading to a confusing user experience.
To avoid the coupon field from showing up
1. Go to Checkout Flows (/admin/commerce/config/checkout-flows)
2. Edit the Default Checkout Flow (/admin/commerce/config/checkout-flows/manage/default)
3. Disable the 'Coupon redemption' Field by dragging the field to the disabled sections
Clear the Cache using Drush (Cache rebuild = cr)
drush cr
or by going to the Performance section (/admin/config/development/performance)
Now the Coupon Redemption won't show up in your default Order Form.
Drupal