you are probally too low level for your charmander. what lvl are you, and what lvl is your charmander?
If you are on gen 3 or 4, you need badges to use pokemon up to a certain level, look in your /pokegear and look in the center for the badges. gen 3 badges go up and down as Gen 4 badges go to the right(for the order you need to do the gyms in) Once you get the badge required to the level of your charmeloen then you will be able to use the pokemon again! Further questions your welcomed to contact me here or on our discord DarkEclipic#3032 , I hope this helped <3