Help with "Non-RGB" error message

I just downloaded fjij, so I have no idea how to use it. I am trying to find the radius of curvature from a picture, so I googled how to do that.

I found some code and ran it by doing Plugins>>New>>Macro, and then pasting the code in there. When I hit run, I got the following error message: “Non-RBG image expected in line 7 setAuto Threshold (“Minimum dark” <)>;”. Then there was a line below it that said “Show ‘Debug’ Window” with a box next to it that I could click.

I will paste the code I am using below. If someone could help me figure out how to fix the error, that would be great!

// imagej-macro "curvatureRadius-2" (Herbie G., 05. December 2018)
requires( "1.52i" );
setOption("BlackBackground", true);
run("Find Edges");
setAutoThreshold("Minimum dark");
run("Convert to Mask");
run("Create Selection");
run("Area to Line");
run("Fit Circle");
getSelectionBounds(x, y, w, h);
print("Radius: "+d2s(w*0.5, 1));
// imagej-macro "curvatureRadius-2" (Herbie G., 05. December 2018)

It does help to indicate where you got the code from, and to format it as code - I have now edited your post.
The original post: How to calculate the radius of curvature for an image of an arc - #5 by anon96376101
The original post shows a grayscale image - and that is what the setAutoThreshold will run on - your input image for that step needs to be non-RGB. Depending on your source image, it may be as simple as changing the Image->Type->8bit, though it is hard to tell whether that will be effective without a sample image.

Additional note, the crop step was also for the sample image provided by the poster, and may not apply in your case. The script was for that image, not for a general case.