First: Add findByEmail

package com.sportsbuy.authservice_microservices.repository;

import com.sportsbuy.authservice_microservices.entity.User;
import org.springframework.data.mongodb.repository.MongoRepository;

import java.util.List;
import java.util.Optional;

public interface UserRepository extends MongoRepository<User, String> {
    Optional<User> findByEmail(String email);

}

Second: Use findByEmail in controller

public String createUser(@RequestBody User user) {
        String email = user.getEmail();

        Optional<User> user = userRepository.findByEmail(user.getEmail());
        if (user.isPresent()){
            return "exists";
        }
        else{
            return "NO exists";
        }
}