Error "Transparent encoding with auto_alt_ref does not work" when converting .mov with Alpha to .webm with alpha with ffmpeg

I am trying to convert a .mov file with alpha transparency to a .webm file and am following this thread: Converting mov with Alpha to VP9 Webm with alpha Using ffmpeg

On the command line, I use

ffmpeg -r 24/1 -i Desktop/Skel_Walk_1.mov -c:v libvpx -pix_fmt yuva420p Desktop/Skel_Walk_1.webm

      

        
        
        
      

    

However, when I go to run the command, it comes up with two errors

Transparency encoding with auto_alt_ref does not work

      

        
        
        
      

    

and

Error initializing output stream 0:0 -- Error while opening encoder for output stream #0:0 
- maybe incorrect parameters such as bit_rate, rate, width or height

      

        
        
        
      

    

I'm not too sure what the problem is here, any suggestions, I'm very confused!