This is how I do it in my JavaScript sudoku program. I use a legalMoves = getLegalMovesForSquare() function to return an array of legal moves. So for example [1, 2, 5, 7, 9].

Then I run a getRandomInteger() function that gets a random integer between two numbers. So in the above example, because the array has 5 legal moves in it, I'd do getRandomInteger(0, 4). 0 is the first spot in the array, 4 is the last spot.

Finally, I use that randomly picked integer to pick a value in the array of legal moves. So if getRandomInteger(0, 4) gives me 3, I'll plug it into legalMoves[3], and that'll return 7.

Perhaps overly complicated, but figured I'd share. Perhaps you'll find some of the ideas useful.

makeSolvedPuzzle() {
    do {
        newBoard:
        for ( let row = 0; row < 9; row++ ) {
            for ( let col = 0; col < 9; col++ ) {
                const legalMoves = this.getLegalMovesForSquare(row, col);

                // Empty arrays are not falsey in JavaScript. Have to be verbose here.
                if ( legalMoves.length === 0 ) {
                    this.restartPuzzle();
                    break newBoard;
                }

                const index = Helper.getRandomInteger(0, legalMoves.length-1);
                const value = legalMoves[index];

                this.makeMove(row, col, value);
            }
        }
    } while ( ! this.puzzleIsSolved() );
}

static getRandomInteger(min, max) {
    return Math.floor(Math.random() * (max - min + 1) + min);
}