For a more in-depth explanation:
The NVENC encoder is limited to 2 instances on anything that isn't a Quadro card. Windows Game DVR and Gaming features is using up an instance of the GPU encoder, and so is shadowplay. OBS needs to run an instance too. So to give OBS headroom, it's recommended to disable Windoes Gaming features
and here is how to do it.
As for the reason to not record in 240 fps:
1. x264, NVENC and AMD VCE isn't optimized for 1080p240 recordings. It is not impossible, but impractical.
2. For that impracticality, it is mostly a codec specific limitation. You would need to record at "level 5.1 or 5.2" to achieve 240fps with H.264, which is most certainly possible, but...
3. It would require a very large bitrate to do so at a decent quality, and can definitely tank a standard harddrive, and potentionally a standard SSD. NVMe SSD's are probably safe for this, but they are costly.
4. Many if not most video players will not play H.264 (incl. x264) video files encoded with level 5.1 or 5.2. And even less video editors can work with them.
Lastly, most video services online (youtube, twitch, vimeo and daily motion, to name a few) will only play back the video at 60fps anyways.
That is why, if you really need high framerate recordings, I recommend 120fps at most. Or record to HEVC, which even less video editors can work with.