This can be due to some syntax error(s) in the way you wrote your query.
Here are the most common errors that you should correct:
You cannot start a query with AND all uppercase. AND is reserved as a search operator.
You cannot start a query with OR all uppercase. OR is reserved as a search operator.
You cannot start or type a query with only one double quote. Double quotes are reserved as a search operator for exact phrase.
You cannot start or type a query with only one parenthesis. Parenthesis are reserved for combining operators.
You cannot start or type a query with only one forward slash. Forward slashes are reserved for regular expressions (Regex).
You cannot start a query with tilde (~) or write one which contains tilde. Tilde is reserved as a search operator for fuzziness or proximity searches.
You cannot end a query with question mark (!). Question mark is reserved as a search operator for excluding a term.
You cannot start a query with caret (^) or write one which contains caret. Caret is reserved as a boosting operator.
You cannot use square brackets except for searching for ranges.