Fleas

@artkid19, sometimes when a dog or cat in The Sims 4 appears to have fleas , they are really a symptom of an illness.  In this case you would have to take them to the vet.  When you click on your pet, there should be an option as soon as you click to take them to the vet if they ate sick, which usually does not show up if they aren't sick.

I have not encountered just plain old fleas in the game yet, so I don't know if they even have them.  🤔

Does you Sim's dog have an option to go to the vet that is given at the beginning of the menu when you click on him/her?

Go to the vet and from the vending machine purchase cat wellness treats (i buy a few at a time). I have used these to fix the flea/fly problem for my sims pet cats and for strays that show up bluish and unwell. It works!

Treat them w the cone of shame. 

@TheNxtStep05, yes, the "cone of shame" is one treatment that will take care of the problem 🙂, but you still have to have your Sims take their pets to the vet clinic for this, as was stated above.  The cone will be the cheaper treatment, but you can also get them the more expensive treatment and this will also work.  Then there is also the Wellness Treats that @Juliaj  mentioned.❤️  But this is an older thread (from almost 2 years ago) , so it isn't likely that @artkid19 still has this problem. 😉