clearstream
(He, Him)
There are some things that a character cannot meaningfully try. The best approach is to deflect them with an indirect "No", by asking "How will you do that?" That directs their thought to something they genuinely could attempt. So a 1st-level Half-Orc Fighter cannot "attempt" to cast a fireball. They can attempt to learn where an item might be found that could give them that possibility. But then, what they are attempting is something else... they are attempting to find the local sage perhaps... or whatever act seems likely to set them on the right course.I disagree.
Anything, no matter how ridiculous, can be attempted.
The DM can tell me my PC has absolutely zero chance of success and might even hurt or kill itself in the process, but she's overstepped her bounds if she tells me I can't make the attempt.
Put another way, there's a very big difference between her telling me I can't succeed and telling me I can't try.
The problem I see with "Go ahead, but you can't succeed" is that I think this really is just to say "You cannot attempt that" but it's also not constructive. It doesn't guide the player to think of something their character really could attempt. It might be fun a few times for comic relief, if the half-orc goes through the motions they have seen the party Wizard doing. But that could also mislead and mock the player if they genuinely didn't realise that casting fireball is not something a 1st-level fighter can do. I think it would have been better to let them know that they might prefer to roll a wizard or sorcerer who will be able to do it at level 5.