I am almost at my wits end with the parry issue, playing on Linux. Sometimes it seems like every 1/20 times, even when timing the parry perfectly, it does not register. This is particularly frustrating on bosses that hit very hard and you either parry or you die. I don't want to install a mod, but I'm not sure what else to do at this point. It's incredibly frustrating.

ok

maybe the framecap should be 33 fps...idk

I do not advice to use the mod from the previous post that makes dodging/parry easier. The parry and dodge method is by design meant to be a challenge and implemented in a way, that involves learning and getting attached to enemy attack patterns, hence it is part of the gameplay-loop that makes this game so well made. However, it is essentially required to dodge and parry, because you can die very easy if you don't which is also part of the design choice that encourages to learn the mechanics. By removing that, the gameplay-loop becomes quite boring and there would be no meaning left in learning enemy attack patterns. You could as well just use an invincible cheat and that would give you almost the same outcome. That'll spoil your gameplay fun. Not recommended.

I can confirm, this has indeed helped to reduce the input problems (for the most part as far as I could observe). Sometimes the game won't respond to your input, and capping framerates to have a stable frametime helps. However, as previously mentioned from other posts, it's best to use RivaTuner to cap frames instead of VSync or the in-game FPS Limiter. I also specifically advice not to use the in-game Frame-Limiter (keep it at unlimited), because it causes the frametimes to be unstable. Use RivaTuner instead:
https://www.guru3d.com/download/rtss-rivatuner-statistics-server-download/

Also you may want to use this mod, to further improve responsiveness:
https://www.nexusmods.com/clairobscurexpedition33/mods/4

Or if you are old like me and want a longer action window to parry / dodge without changing the difficulty to easy, just use this mod :

https://www.nexusmods.com/clairobscurexpedition33/mods/28?tab=files

so what is best people say different?

You would be better off just setting a frame cap via either the in-game one or through the drivers, or a third party app like RivaTuner.

Using V-sync ONLY to cap the fps is not a good idea, especially in this case, considering it will add more input delay.

weird i allready had v-sync on and had framecap set to 60 could easily run higher but thought it might fix it.

Yes in combat even if i press "W" for a skill or "A"/"D" to switch targets,
it sometimes ignore the input.
Will test it today with XBox controller.