The ATC error messages use "ERROR: Tool confliction occured, please check tool rack!" as the error message for when the tool sensor detects a tool already in the rack and when there is not a tool in the rack to pick up. This is a simple clarification of that information
faecorrigan
commented
6 hours ago 
The ATC error messages use "ERROR: Tool confliction occured, please check tool rack!" as the error message for when the tool sensor detects a tool already in the rack and when there is not a tool in the rack to pick up. This is a simple clarification of that information
based on this request from the brooklikeme repo:
https://github.com/brooklikeme/Smoothieware/commit/695a2499217f8f42ce5436beea415b5857d0f7a8#diff-cffcd3ae63a965f7278f81fe46497b8a30bf5d0518f9f38b9c31156af14d0222L843