Hi TTD,
Unfortunately it isn't the easiest machine to understand. You really have to just play, play and play some more to get a feel for how it operates and what settings work.
The user manual is pretty much useless, so you'll need to just try things and ask allot of questions. The forum has been pretty helpful for me but there's not allot of mimaki operaters compared to other
dtg users out there. I think i am only 1 of 4 in Australia with a mimaki gp-604D from what the sales rep told me 12months ago.
Anyway if you have any questions maybe i can try and help but I'm still trying to figure this machine out as well.
To your question
"ERROR 120 ENV TEMP LOW" you can either get a LOW or HIGH error reading, in your case its low. You need to adjust your temp setting in your profile settings to a more suitable temp for your space or environment. Temp setting should be set depending on your type of print and garment thickness. For tshirts i usually just keep mine at 45-50 deg temp.
Try and search through previous post there is some good info for you on the forums to read about the mimaki.
Hope this helps, but please feel free to ask me questions if you want. Also there is a mimaki rep here on the forums,
stevek517 he is located in the U.S but he has helped me here in Australia via the forum and email with advice. Maybe try asking him for some help. Also before i go, Did you buy the machine new and what made you buy the mimaki??
cheers
Andrew
error 120 evm temp low 
Did you buy it from DGS Mimaki Australia? May I also request you to answer Bobbielee's thread here because I noticed that there's not much feedback from Mimaki users here: 
Heat Transfer Paper Price List
