Iris Shaders Crash: java.lang.AssertionError: Can't. (iris$getRenderTarget)

What the log looks like

java\.lang\.AssertionError: Can't\.\s+at [^\r\n]*iris\$getRenderTarget
java\.lang\.AssertionError: Can't\.[\s\S]{1,250}net\.irisshaders\.batchedentityrendering\.impl\.FullyBufferedMultiBufferSource

How to fix it

1. Update Iris Shaders and Sodium to the newest available version for your Minecraft release, as patches after Iris 1.9.0 improved compatibility with non-standard render layers.
2. Remove the Essential Mod from your mods folder if installed; its player cosmetics and multiplayer overlays frequently trigger this exact Iris render-target assertion when rendering players in F5 or multiplayer.
3. If you rely on Essential to open singleplayer worlds to friends, replace it with a lightweight, Iris-compatible LAN hosting mod such as e4mc or World Host.
4. If Essential is not installed, check for and remove other client-side mods that add custom 3D cosmetics, capes, emotes, or entity outlines, or toggle shaders off before switching to third-person view.

Check your own log