Everyone has difficulty with spelling from time to time. You might make a mistake when you use a word infrequently, or have trouble reporting a word’s spelling verbally when you’re put on the spot. These are common issues for a few reasons. One is that spelling is something we usually do in writing - delivering this information in another modality can be awkward. Two, spelling is information that we store in the brain as procedural knowledge.
This means it becomes automatic only after a person builds up extensive contact through repeat exposure to a word in reading and writing. You might have certain words you always mix up because you never learned the correct spelling, or because you wrote them incorrectly and now can’t tell the right from the wrong version. But some people struggle with spelling in a very different way.
For example, dyslexic individuals may put a word’s letters in the wrong order, miss out on a letter, or even add one that doesn’t belong. They often have difficulty spelling consistently, and can get a word right one day and not the next. This doesn’t mean they are “stupid” or “lazy,” it just means there’s something interrupting the encoding process by which words are split into their component sounds and those sounds are then mapped onto English letters.