This appears rarely on 1.16.5. Download MixinBootstrap (for older Forge) or update to the latest recommended Forge version for 1.16.5 (36.2.34 or newer).

To run most web-based mods in Minecraft, you typically need a specific framework to let the game "talk" to the internet: Install Forge : Most display mods require the Minecraft Forge mod loader. Download MCEF : This stands for Minecraft Chromium Embedded Framework