Whenever I try to add a rider to one of my horses I get an error message. No rider is added yet the horse no longer appears on the hire rider page and it says all horses have a rider assigned

I have included a link to a screenshot to show you

Screenshot

Not a bug. The horse is a Schoolmaster in all disciplines so you can't hire a rider for it.

So how do I show it or train a rider?

From the game guide, where you can find such helpful informations:

Horse Retirement

Once a horse reaches International Level in a discipline, they can continue to show and train for a while. After they reach a certain point, they will be automatically retired by the game and convert to Schoolmaster status for that discipline. They will no longer be able to show and train in that discipline.

However, you may use your Schoolmaster in Rider Training, to boost the number of training points a Rider gains when the Rider trains. You also may choose to change your Schoolmaster's training discipline to train/show in a different discipline.