I'm pretty sure anyone can walk a day.
I'll assume you mean dog. And no, they cannot. Or at least they cannot safely in a commercial manner. Which is exactly my point.
They can, of course, hang onto a lead while a dog is on the other end. But that's not the kind of service I would pay for.
For example, I expect any walker I hire to have first aid qualifications, in case of accident etc and to know where all the local vets are and to keep good, easily accessible records on which dog is registered with which vet.
To have qualifications in the basics of canine body language to be able to adequately read the dogs they are walking and any they encounter. To be able to combine the dogs (energy, play style etc) so that all dogs fully enjoy their walks. I've come across far too many pet owners who think they know dog body language but whose interpretations of a scenario are way, way off.
Bonus for also have good training knowledge to be able to have a positive influence on the dog's long term walk behaviours.
I would expect them to be aware and up to date on dog crime tactics so as to reduce the risk of being targetted for dog thefts, including things like the use of unmarked vans etc. And to keep up to date on the frequency/likelihood in their local patch.
I would expect them to have reasonable knowledge of the local area, including good places to walk in different weathers. My own dog walker knows all the quiet, stream-side spots that are under shade, for hot days. She knows which areas get especially icey in winter to avoid - especially for elderly or less robust dogs. She knows when places are busy and when they are quiet, when farm work is going on, when sheep are in fields with public footpaths.
She knows the law around dog walking - which many pet owners do not. Things like collar requirements and what constitutes a dangerously out of control dog (too many people do not know and just assume it means an aggressive dog).
Does a pet owner know and have all this? Sometimes, yes. Often, no.