Whenever someone says this to you:
"You can't do that!" (or some variation like, "That won't work.")
It usually means they're only saying:
"You can't do that the way I'm thinking you'll have to do that in order to do that."
The problem is just that they haven't seen that done in that way before.
And of course, they could be right. So learn from but don't be limited by the experience of those who've gone before.
Because maybe you really can do that, even if no one else has ever done it that way before.