The 3CX is updated to the latest version (V18 update 5 build 418)
Logged in with the Admin account
But the add whatsapp fuction is not visible in 3CX (web)
If I understand your question, you don't interact directly with WhatsApp. Rather, you respond to inbound text messages the same way you would with any other text message. If you need instructions for setting up the WhatsApp integration, I held a webinar a few weeks ago where I walk you step-by-step through the setup proccess. You can see the YouTube video here
If you are using the Startup edition of 3CX, the process is different than the dedicated Pro and Enterprise editions. The Whatsapp setup documentation for 3CX shows you how to do it for each version. The configuration screens are different.