Dogs can safely eat small amounts of cooked or boiled spinach but don't overfeed the dog too much spinach as too much spinach for dogs can be harmful.
Also raw or uncooked spinach when fed to dogs is hard to digest which can cause upset stomachs, vomiting, diarrhea etc in the dog.
If you want to feed your dog some spinach it's okay in moderation as long as you cook or boil the spinach before feeding the spinach to your dog.
The cooked or boiled spinach will be easier for the dog to digest.
Large quantities of spinach when fed to the dog could cause kidney problems but if your dog already has kidney disease or kidney problems you should not feed the dog with kidney disease or kidney problems spinach because it can cause more problems.