A site is an alternate address to your contacts physical address. Typically used if your have a contact that has multiple addresses that you might visit for work.
Part 1: How to ensure that adding sites to Jobs is enabled
Please note: You can skip 'Part 1' if you already have this option enabled in your Settings.
Step 1: Select Admin then Settings
Step 2: Tick Allow defining Sites for Jobs and Tasks then Save
You can find this under the Jobs and Tasks section.

Part 2: How to assign a site to your Job
Please note: We are assigning a Site against an existing Job in this example but you can also assign a site when creating a new Job. Also note that the same steps apply for assigning a Site against a Task.
Step 1: Navigate to All Jobs and select your Job

Step 2: Select Edit
There is no Site currently assigned to this Job.

Step 3: Select Site

Step 4: Select your Site Contact
Please note: You will only see your Site Contact here if they are linked to the contact assigned against the Job. Follow this help article on how to link contacts together - Linked Contacts

Step 5: Select Save

