1. Make sure the membership level has "Allow signup for this membership from website" enabled under Labels & Membership > Membership Levels.
2. Make sure the billing options under the membership level has "Allow members to select this billing option" enabled.
3. If you had specified a credit card billing method for the billing options/membership level, make sure that you have enabled a payment gateway under Organization Settings > Integration > Payments.
4. Replace the shortcode/HTML snippet on the sign up page. Make sure that if you use the "Optional - display only specific levels ..." feature, that you include the membership levels you want to appear.