Some autistic children can seem impossible to help. You can see your child struggling but any offer of help is disregarded or met with a meltdown. This can be hard for neurotypical people to understand and can make parents and carers feel as if it is a personal rejection.
Autistic people often have very clear ideas when they embark on an activity and someone else getting involved is not in their plan. It often goes far deeper than that. There is the fear that any other person will not share the same end goal. This means that any help can be perceived as the polar opposite and not helpful at all. The help doesn’t improve what they are trying to do because they worry and get stressed that the other person is trying to achieve something else completely different.
This doesn’t mean that there are not ways to support the child. The best way is often to watch and observe what they are doing and wait for cues that they need help. You have to be patient and you can say "I'm here if you need me". But never assume you know what the autistic child is trying to achieve, because it may not be what you think.