Hi @redafillali,
Thanks for reaching out!
I understand that when you applied a fixed amount discount coupon to an order, the price becomes zero (0).
Can you please share how you generated this coupon code? Is it thru Marketing - Coupons - Add New
?
If so, you can set specific restrictions for this coupon based on your requirements under the Usage Restrictions
tab here.

Image Link: https://snipboard.io/G1eldH.jpg
You can also check our support page below to help you set up the coupons properly:
https://woocommerce.com/document/coupon-management/
For info : The cart minimum is 300 and coupon can never be more then 50% of the cart total
Based on your example here, it seems that you want to use percentage discount coupon
instead.
You can try the settings below on your site for testing purposes. Here, I created a Percentage discount having 40% when the minimum spent is 300. Of course, you could also add other restrictions as well.

Image Link: https://snipboard.io/xtTiP3.jpg

Image Link: https://snipboard.io/rUYbJw.jpg
Otherwise, are you using a plugin in generating this coupon? If yes, can you please share what plugin was used?
Thanks!