In RAW Mettle the assumption is that certain tasks can just be out of range for a character whose pool is too low. This does not happen often as the max Score is actually three times the die pool.

You can open up the Score to a practically infinite level, depending only upon your patience and available dice. Simply rule that as long as there is no Edge, the player may add a die. As soon as they get Edge, they must stop. The single point of Edge still counts, even for Twists.

This can be useful for tasks that may always have the tiniest chance of success, even for the unskilled. It can be a gracious act for the Guide to grant this to an outmatched character facing incredible odds, but should probably be an exception rather than a rule.