Why I can‘t see Phoenix and Charlotte anytime I get to the city center after I ask Phoenix to be the bodyguard of Charlotte.

in  the yacht? probly

is random to happen after the quest, there's a chance for them to be together at the yacht or at the boutique at the mall

After you went to the gym and asked Phoenix to be Charlotte's bodyguard, go to your Home Entrance and you'll get a scene where you introduce Phoenix to Charlotte. 

Then you can go to the Mall and you'll see that Phoenix do a great job as Charlotte's bodyguard. After that, go to the Yacht on Monday and make sure to fuck Charlotte in the ass to get Spitroast scene with Phoenix ( Repeatable ). 

Last but not least, you can meet Phoenix and Charlotte shopping in Boutique Store and have a threesome scene ( Repeatable )