It’s this part:
LIST *start = head;
The parameter for the function is a pointer to a constant, const LIST *head; this means you cannot change what it is pointing to. However, the pointer above is to non-const; you could dereference it and change it.
It needs to be const as well:
const LIST *start = head;
The same applies to your return type.
All the compiler is saying is: “Hey, you said to the caller ‘I won’t change anything’, but you’re opening up opportunities for that.”