I have an ai. document and set it for RGB color mode. I then picked the bright green (r=0, b=0, g=255) and it says image is out of gamut and won't print properly. There must be something I'm missing but I can't figure it out. I have been using this program for years and never ran into this problem. Can someone please help me.
That's the point of the "Out of Gamut" warning. It is telling you that printers can't print the color because it's out of the gamut of your CMYK printer. For example, a nice blue on your monitor will print as purple if it's out of gamut.
Illustrator has its own color management system that uses your printer profiles to automatically bring all colors into gamut that will print to that profile’s range of colors. You need to identify the out-of-gamut colors in an image or correct them manually before printing to your desktop printer or converting to CMYK for a professional print job.