You should give more info. Are you putting them in the /plugin folder with the correct titleid? Are you sure your NTR CFW is working correctly? Are you on old or new 3DS?
I have putting them in right plugin folder, i'm using Boot NTR Selector- Enhanced NTR CFW Loader and I have putting them in plugin folder so was made after start up Boot NTR Selector.
I have old Nintendo 3DS
Old 3DS has to use the Boot3 mode of NTR, and you have to wait a few seconds after startup. Codes also won't work for extended memory games, like Smash bros. You'll know if a code worked if the top screen flashes green when the game starts up.
Ok I'm going to try Boot 3 NTR.
So I have download Boot mode 3 NTR but then i get this error.
You can see in picture what error i get and what homebrew i have download.