If you are using Optimizepress and you want to have a WhatsApp Widget working you can not use plugins for WhatsApp inside WordPress. You can use a script to insert the WhatsApp Widget.
You need to create a free account here to have a WhatsApp Widget.
Once you have the whatsapp widget created, you can copy and paste the script in the section available for that inside your Optimizepress landing page.
This is a workaround that should help you creating your WhatsApp Widget inside Optimizepress website.