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Go to Marketing -> Cart Price Rules From Navigation
Add New Rule from Cart Price Rules Magento Admin page
Fill out below *Required fields details of cart price rules
Rule Name: Enter your Rule name Like Monsoon Sale , Summer sale etc
Websites: If stores have multiple website than select specific website.
Customer Group: Select Group who can use this rules.
Coupon: Select No coupon or Specific Coupon
Coupon Code: if selected Specific coupon than give unique coupon code.
You can write condition for apply rules for example in cart specific category product compulsory etc.
Configure Action Section of cart price rule.
After that Save Cart Price Rules.
Now customer apply coupon code on cart and checkout page and they get discount according conditions of rule.