Synopsis
In calling some function, the option arg you specified was not one of the supported choices. An option arg is a single letter, for example "L" selects the "trim leading spaces" option for the STRIP() built-in function. name is the function which complained about the bad choice. number tells which arg has the bad choice. choices are the allowable letters for options. bad choice is what you supplied.
Cause
You wanted to specify your arg as a literal string, but you forgot to put quotes around it. Therefore, Reginald assumes that it is the name of a variable which contains the real value. And that variable was previously assigned a value that is not just one of the allowable choices. This is likely the case if bad choice is not what you intended.
Cure
If you're directly supplying your arg as a literal string, put quotes around it.
Cause
You stored the option in a variable, and then when passing that, you put the name of that variable in quotes, thus mistakenly passing the variable's name rather than its value. This is likely the case if bad choice is your variable name.
Cure
Do not put your variable name in quotes.
Cause
You specified a variable name, but that variable was never assigned any value. Therefore, its default value is its name in capital letters. This is likely the case if bad choice is your variable name in capital letters.
Cure
Make sure that you assign a value to the variable before calling the function. You can trap the NOVALUE condition to catch errors of this nature.
Cause
bad choice is indeed the option you want. But, the function doesn't support this (ie, the choice you want is not listed under choices).
Cure
Update to the latest version of the software containing that function.