I think there may be a problem with beginning your record and not ending it in ur draw loop.

1. void setup(){
2. //set up stuff
3. noLoop();
   
4. }
   
5. 
   
6. void draw(){
7.       if(saveOneFrame){
8.             beginRecord();
   
9.       }
10.         background(255);
            translate(mouseX/2, mouseY/2);
            font.draw("Hello?");
11.       if(saveOneFrame){
12.             endRecord();
    
13.       }else{
14.             redraw();// create your own draw loop rythm with this else statement that will call re call the draw()
    
15.       }
    
16. }
17. 
    void KeyPressed () {
      if (key == 'B' || key == 'b') { // When 'B' or 'b' is pressed,
        saveOneFrame = true;
      }
    

It might like that better than starting a record session and not ending it within a draw frame

Kobby is right.
Beside, it is keyPressed, not KeyPressed, so if you rely on this function, you never start recording...