Click that Film Reel icon in the lower left corner. That brings up the Mantra Player, MPLAY. Choose File/Export/Quicktime from the MPLAY menu. That is how you can export a rendered image sequence into a Quicktime or Windows video format.
Using Houdini Indie 20.5 Windows 11 64GB Ryzen 16 core. nVidia 3060RTX 12BG RAM.