mkv not an option for output of recording

[Univarseman]

I really want mkv because it leads to less issues when recording and makes for a better safeguard in case the program hardlocks or the system gets interrupted for whatever reason

 

[PaiSand]

As you selected custom fmmpeg output the name may be different.
In this case, matroska is what you want. MKV means matroska video.