Anvil embeds reference to a script in your 3ds Max scripts folder. If that file is missing, there can be an error.
Anvil embeds some scripts for scene startup to protect displacements from being invalidated via improper manipulation. Older versions of Anvil referenced a file that could cause an error if missing.
This error usually indicates Steam did an update or you are working with a mod that doesn't use the Steam VProject setting. Read the fix here.