java.lang.NoSuchMethodError: com.obsidianclient.asm.Hooks.onGuiOpen(Lnet/minecraft/client/gui/GuiScreen;)

Did you install Obsidian Client for vanilla Minecraft or did you use it with Minecraft Forge?
And please provide the full log output. (If you are using the standard Minecraft Launcher, go to settings -> Minecraft: Java Edition settings -> open output log when Minecraft starts.)