Okay, so yes, that's wrong. That's an msys2 style path which Windows does not understand.
Okay, so hit the windows key and start typing "edit environment variables", and you should get a result, open it.
From there, you should get two lists of environment variables; one specific to your user account and one to your machine. The variable names are listed in the left column of each list; look through them and find the entry for DEVKITARM. I don't know which list it'll be in - if I had to guess, I'd say the system variables, but I'm not sure. Just look through both until you find it.
Once you find it, select it and then click Edit. Then change the value from /opt/devkitpro/devkitARM to C:\devkitPro\devkitARM.
Also, while you're there, you're probably going to want to set up the ARMIPS environment variable. Hit New , give ARMIPS as the variable name, and put the absolute path to armips.exe. For example, if it was C:\Users\eMM97\Documents\armips.exe , you would put the variable value as C:\Users\eMM97\Documents\armips.exe.