Sorry for facing this issue, it's my pleasure to help us.
Checkout page customization allows only for Shopify plus plan do you have this?
If helpful then please Like and Accept Solution. Want to modify or custom changes on store Hire me. - Feel free to contact me on email@example.com regarding any help Shopify Expert | Skype : bamaniya.sky PSD to Shopify | Shopify Design Changes | Shopify Custom Theme Development and Desing | Custom Modifications In to Shopify Theme | SEO & Digital Marketing