For SVRT to be an option and tickable, your output video specifics, (frame rate, frame size, bitrate) all need to be essentially the same as your input video. If not, it will not be an option. You can try to use the "Intelligent SVRT" button at the top of the format specification area which will analyze the footage and suggest a produce option for SVRT. If it has a suitable format, then you can apply that suggested format and the option should then be available.

All that said, everyone’s "happiness" with SVRT is in the eye of the viewer, it does not do a good job of doing what the docs say it does without introducing artifacts (audio, video, frame skips, black frames...), all depending on TL complexity and video source, subtle to some, a huge negative to others. I'd suggest you simply turn off all HA and let the CPU's do the encoding for most likely the best video quality results.

Jeff

Thank you for replying, Jeff.

It's now clear for me. I didn't choose the good profile, and now it works as you explained. Jean-Paul

i7-4770K 3.50 GHz, Nvidia GTX 960 2 Go, SSD 500 Go, RAM 32 Go
Win 10 64 bits, DirectorSuite 365