Hello and welcome!
You can either assign some temporary code column, or just re-use the variable name instead of the code column itself. Usagi should not complain about this.
As for Linux compatibility, Usagi only depend on Java being installed. Unfortunately, it can be picky about what JRE provider to use – these days there are multiple open source implementations, good old one from Oracle and even one from Amazon. Usagi explicitly requires Oracle’s distribution, and I personally had problems using OpenJDK. You will have to check which package you have installed, for example, by running
java --version in the terminal. Unless you see “Oracle” in the output, you need to reinstall.
Otherwise, run Usagi from the terminal as documented in the last section here, maybe you will get some useful error messages.