Guys, if you have not read the manual, you are not doing yourself any favors. This ain't Candy Crush.
Invalid entries are highlighted in the left side input area with an exclamation point (!) inside a diamond shaped box. If you hover over the input area for that invalid parameter, a message will appear at the top that will tell you what is wrong. What it says might be less than what you need to know why because the program assumes you understand all the parameters used and you may not understand this particular one. I get these messages every so often because I use zero seating depth for breech seated bullets and the program is expecting fixed ammo. But it will still use zero or even a negative number for bullets seated beyond the case mouth and still run the calcs.
You might not have chosen a powder or have too small a powder charge. You need to look at all the cells, understand them and for the ones that the program does not like, figure out why they don't like it. It is normally pretty obvious once you take it cell by cell.
The first day I started using the program, I thought it was pretty easy. But then I started getting invalid entry errors and realized I needed to be more involved in the input than just picking caliber, bullet and powder and seeing what it spits out. The program also makes some assumptions and as such had default values it plugs into some cells. They may not work with what you are doing. It will take some time working with it, referencing the manual when you are stuck and experimenting with inputs to feel comfortable. But, it is by no means a difficult program to use.
We are all wired differently. I go thru this every day with my wife. What can be blatantly obvious to me can be totally invisible to her. It's nobody's fault and it is not a reason to criticize. So, I will say this and it may help. If you have poor reading comprehension GRT might be difficult for you to use. If you are totally baffled by electrical diagrams and even when explained, you are still overwhelmed, then this program may not be something you will quickly learn and understand. That does not mean that you won't eventually. But it may take more effort and possibly pain and frustration on your end before that happens. I think your desire to learn how to use the program because you have a need and want to understand more about internal ballistics will determine the pace at which you will learn to use it. If you are simply curious and only want to dabble in this stuff, then my guess is you won't have the true initiative to work past the challenges and frustrations to figure it out. And these comments are only made because I am fully aware how different we all are when it comes to how our brains function. I still maintain that this program is not difficult to use if you take the time to understand what is in front of you, on the screen.
If you have specific questions about some of the variables, I suggest that you post them here. I'm no expert on GRT but have been using it for about 2 months now. If I cannot answer your specific question, perhaps other members familiar with GRT or ballistic variables in general can provide an answer. If you have been following my thread on my .38-55, you'll see I'm learning along the way. I would think that this thread could become beneficial to all as more start to use GRT to develop loads.