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.
To enable Customer Accounts, go to Settings > Checkout from your dashboard, then click Edit in Advanced settings. Scroll down to Checkout Options and check the box for Customer Accounts, then save.
Note: The Customer Accounts option isn't available if you're using the Membership feature. If you want to use Customer Accounts, you can disable Membership by removing all members and member groups in Settings > Members. You will also need to change the visibility for any pages that were visible only to members or member groups.
Q) After customer creates a username and password for the membership area they are getting a Forbidden error like the one below, why?
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.