No. Writing fanfiction involving sexual content about a real person is inappropriate and unethical. It violates the person's privacy and can be considered a form of objectification.
One great fanfiction could be about them going on an adventure together. Maybe they find a mysterious object and have to figure out what it is while also dealing with their growing feelings for each other.
There's no conclusive evidence that they are in a romantic relationship. They may be good friends who have worked together and have great on - screen chemistry, but that doesn't necessarily mean they are in love.
You can write about her character in a show or movie she's in, focusing on their adventures, friendships, or growth. For example, if she played a superhero, you could write about her heroic deeds and how she saves the day.
There is no publicly confirmed real - life love story between Leo Howard and Olivia Holt. They are actors who have worked together and might have a great on - screen chemistry, but as of now, there's no evidence of a romantic relationship off - screen.
Well, you could create an adventure fanfiction where Leo and Olivia are on a quest to save a magical land. There could be lots of action and mystery involved.
You can start by looking on fanfiction websites like Archive of Our Own (AO3). There are often many stories there written by fans. Just search for their names and you'll likely find a good selection.