What Operating System are you using? Windows, Linux, Mac? Which version?
Windows 7 home premium Service Pack 1
Which ERA version are you using? Just in case, bear in mind that the Arcane Companion requires the latest ERA update (2017-01-21) from RPGNow.
I looked in my library and saw a 2FEB17 version (was using 8DEC2016) so I downloaded that one and got two slightly different error messages (screenshots added below)
I also went back through and uninstalled and reinstalled ERA. The order of Import was Arms Law, Character Law, Spell Law, and then Shadow World this is where I got the error this time. It was Arcane Companion last time. This time when I reinstalled, The order was Arms Law, Character Law, Spell Law and then Arcane Companion...and Arcane companion installed just fine.
Check to see if there is a file named ERA-Support.log in the Server directory of your installation. If there is, you can mail it to era-support.voriigkye@recursor.net so that I can get a look "under the hood".
Sent.
If possible, add a screenshot of the message you get when you send the mail, that will also help with the troubleshooting
There was no actual Message when I sent you the e-mail (look for my Screen name if the message seems to be lost)