Welcome to the Shopify Community Forums! Thank you for bringing your question here for support. I understand that you are seeing an issue with when you try to activate the buy now pay later option in your store.
To clarify, I believe you are referring to the new Shop Pay Installments option we recently rolled out for our merchants. If you are using Shopify Payments, eligible stores will have the option to enable this feature.
The first thing I would do is do a full browser cache and cookie clear for all time, close your browser, and then sign back into the store as the ACCOUNT OWNER and attempt once more. if you have multiple accounts on the store, you can check which account is the owner under Settings > Accounts.
If that still doesn't work, to help narrow down the issue, you will want to try in another browser (Firefox, Safari, Chrome, Edge) and if possible on another device or internet connection (mobile phone works too!). If for any reason you are still not able to get this issue fixed you will want to reach out to our live support for additional help.
Please let me know if you have any other questions about your pay installments option.