ERROR | [mtoa] [xgenTranslator] Could not find xgen_procedural in search path $ARNOLD_PLUGIN_PATH
happens only when MtoA is first loaded, and it can be safely ignored. It doesn’t cause any problems when you render, and MtoA loads the XGen procedural from the MtoA procedurals folder.
The error is a consequence of how MtoA searches folders when it loads extensions. If you really need to get rid of the error, you can try copying the xgen_procedural library to the MtoA shaders folder.