You can certainly try
"You can certainly try" is a phrase often used by Dungeon Masters, especially Matt Mercer, to indicate that, while there is nothing in the rules preventing a player from attempting a given course of action, that player doesn't necessarily have any chance of success, and the consequences of trying could be very, very bad.
Alternately, can be used to refer to any character build that's very poorly supported, such as adapting an NPC monster with a Level Adjustment for use as a player character race in Dungeons & Dragons 3e.
Examples of "You can certainly try"
- Bringing a sword to a gunfight.
- Trying to shoot the Evil-Superman-Clone with your handgun.
- Playing any kind of Dragon or half-Dragon in pre-4e D&D. (Dragonborn seem to have filled this niche nicely.)
- Playing a monster in a setting not set up for it.
- Playing a low-combat-utility Face in a hack & slash campaign.
- Using a lasgun on a defensive shield.
- Rolling to seduce the Illithid.