I can’t seem to find an option to enable customer portal for this contact.

This might occur if the contact to whom you are trying to grant portal access has a contact type “Vendor”. In order to grant portal access you need to change the Contact type as “Customer”. In order to change the contact type, follow the below steps :

• Go to Contacts tab and select the appropriate contact.
• Click on the Edit button on the top right corner of the contact details page.
• Change the contact type to “Customer” and click Save.