Have you received any emails in regards to the status of Shopify Payments on your account, by any chance? Please be sure to look in your spam/junk folder to be sure nothing was directed there. If you have received some correspondence about it, please reply directly back to that email for the quickest most efficient way to receive further explanation or clarification into the situation.
However, if you have not received any emails about it, the main reason Shopify Payments wouldn’t show up as a payment option is if your store address wasn’t set to a country that is eligible to use Shopify Payments. See here for the eligible regions and accepted businesses.
To check on this, please go to ‘Settings > General’ and ensure that the Country/Region in the Store address section is indeed set to the United States as seen below:
Once you’ve confirmed that the address is set correctly, you have not received any email correspondence about Shopify Payments on your account and it still is not showing when you go to ‘Settings > Payments’, please reply back here to let me know so that I can look further into it for you.