Why does the option to buy disappear when I remove the discounted price?

My option to buy is not there. for example selling price $20 was $50 so fifty would have a line through it. i don’t want discounted price any more i just want $20 but when i remove $50 and save when i go to my store on the buy now page the option to buy disappears and customers have no way of purchasing my product but if i put $50 back in the option to buy appears again. this is the same for the pay options?