Weebly only allows you to select use Customer Accounts OR Membership not both. You have most likely ticked the option to enable Customer Accounts which disables the membership function on your site.
To disable Customer Accounts, in the Weebly editor go to Store > Setup > Checkout > Advanced Settings. You'll see a box you can check for "Customer Accounts" once removed and saved, you will be able to see/use the Membership Feature again.
Forbidden
You don't have permission to access /store/account/ on this server.
A) This happens when you have enabled Membership Area login and then also enabled Customer Accounts before any members were added. Disable the Customer Accounts feature as per the notes above and the Membership Area and settings will re-appear.
Note: If you can't find an answer to your problem click Here to open a support ticket (requires log in).