One keyword for more than one posts

Hello to everyone!
I have downloaded wordpress yoast by seo, i think it’s a very good plugin!
I would like to ask you something. I noticed that I have to put another keyword in each post.
For example I am editing a site and I have a category Car rental. In the category are included some car models(as posts). In each post I have as a title the car’s model(for example Opel) and as a content the insurance I offer as a travel agent. So i put as a keyword the frase –> car rental and in the page analysis seems that message:
“You’ve used this focus keyword 5 times before, it’s probably a good idea to read this post on cornerstone content and improve your keyword strategy.”
Any ideas what should i put? Every post has the same content and it has to do with car rental and the security we offer as a company.