String functions not working for C language, ISO C99 does not support implicit functions?

Question

I can't get string functions to work in C code, using VS Code with gcc compiler.

#include <stdio.h>
#include <string.h>

int main(){

    char string1[] = "HelloWorld";

    strlwr(string1);
    printf("%s",string1);

    return 0;
}

This is the Error:

Strings.c:8:5: error: call to undeclared function 'strlwr'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    strlwr(string1);
    ^
1 error generated.

Expected the given string "HelloWorld" to print "helloworld" in all lower case.