Nick here from Shopify.
Do you mean the option for you to create a customer account in your admin won't show up?
Or do you mean the option for a customer themselves to create their own accounts on the public front of your store?
If you want your customer to be able to set up their own on your storefront you would need to enable it in your store admin. You can do that by taking the following steps.
You will then see something along the following lines.
Could you let me know where the option isn't showing up for you and if either of the above help?
All the best, Nick
That does seem quite strange @redclover.
Would you be able to list your store URL in a reply to me here please and I can take a look at it and see myself, please? Without seeing the store it's difficult to troubleshoot why this might be happening.
Looking forward to hearing from you.