sceen shot

Hi i just play Minecraft on server or do most survival how i I try to do sceen shot but the pictures of it doesn't pick up and i can hard to explain it but I am confused with it why also I never done sceen shot if getting me also this is in Java 1.15.2 and I did do it also on 1.14.4 and the snapshot 20w22a to make only that get on my sceen shot in the game black sceen shot please i try to explain i don't know better i can say it