Please notice that not all Gigs involves chatting with the client. If it does Click on the Computer choose Freelance then Chat with Client.
You can see if you are supposed to chat with the Client by clicking on the Work tab. All required tasks for the current Gig are listed there.
Correct. However, there is no "Freelance" option on the computer. I've gone through all of the options, and nothing about freelance or chatting with the client is there. And yes, I began looking after being prompted to chat with the client as part of completing the job. However, as there is no option appearing for me to do so (I've also tried clicking on my sim and looking through the cell phone), I'm unable to complete the gig and have to cancel it.
If the Freelancer menu isn't on the Computer and since you say you have no Mods/CC then try doing a factory reset as described in this post: http://answers.ea.com/t5/The-Sims-4/FAQ-Troubleshooting-Steps-amp-Known-Issues/m-p/3621199
