The length of the longest possible word is stored in constant `LENGTH`, so `char str[LENGTH+1];` and `fgets (str, sizeof(str), fp)`. Not sure about how to use `fgets`, but it definitely should get the length of the buffer as second argument. Especially not sure how it handles newlines. I read individual characters instead, which may not be your preference, others used `get_string` from `cs50.h`.

Also, I'm somewhat confused your code works at all. For example, in `insert` and `search`, you don't handle apostrophes. In `load`, for each word you seem to also insert all the endings, so for "cat" you would also insert "at" and "t", which are not in the dictionary.

In `check`, you declare `lower` of length `LENGTH`, while it has to store at most `LENGTH+1` characters (remember the null terminator).

In `newNode`, the use of `calloc` rather than `malloc` already means the memory is zeroed, which means `false` for booleans and `NULL` for pointers (both are actually same as `0` in their value).