I'm getting "Invalid Initializer", what am I doing wrong?

Because you can't initialise like that, you need a constant expression as the initialisation value. Replace it with:

int main (void) {
    char testStr[50] = "Hello, world!";
    char revS[50]; strcpy (revS, testStr);
    // more code here
}

Or, if you really want initialisation, you can use something like:

#define HWSTR "Hello, world!"
int main (void) {
    char testStr[50] = HWSTR;
    char revS[50] = HWSTR;
    // more code here
}

This provides a constant expression with minimal duplication in your source.

Arrays arent assignable.

You should use memcpy to copy contents from testStr to revS

memcpy(revS,testStr,50);

Only constant expressions can be used to initialize arrays, as in your initialization of testStr.

You're trying to initialize revS with another array variable, which is not a constant expression. If you want to copy the contents of the first string into the second, you'll need to use strcpy.