Implicit declaration of function 'random bytes_uniform' is invalid in C99

I am trying to make an encryption script. I am coding this using C in Xcode. To generate a random key I am making use of the randombytes_uniform() function. So this is how I have called it

        int rnumber = randombytes_uniform(10);

However, I am getting this warning which states that

Implicit declaration of function 'random bytes_uniform' is invalid in C99

I have looked through the other questions that have been answered but none of them worked for me. I am running this function in my main. Therefore, I creating header files or declaring functions before the main is rather unnecessary. Even then when I tried it, I still get the same warning.