Command blocks will say the command won't work if it has a slash at the beginning

So if you put in any command and have a slash at the beginning, the command block will say the command won't work.

Like it I type in "/give @p minecraft:diamond", it will say the command will not work, but if I activate the command block, it will still give me a diamond.

Now if I put in "give @p minecraft:diamond", now the command block sees nothing wrong with it and it will give me a diamond. (Notice it doesn't include the slash)