NO - For several reasons. Just need to follow book structure to figure this out.
Page 97 Assassinate - Starting at 3rd level you are at your deadliest
when you get the drop on your enemyies. You have advantage on attack
rolls against any creature that hasn't taken a turn in combat yet. In
addition, any hit you score against a creature that is surprised is a
critical hit.
Paragraphs keep like-minded text together. Since they brought up attack rolls, it is a safe assumption that their intention that they were alluding to 'Attacks' which they describe later.
Page 193 "Making an attack" - Whether your striking with a melee
weapon, firing a weapon at range, or making an attack roll as part of
a spell, an attack has a simple structure.
This clearly limits what 'making an attack' is. Thus casting magic missile or any spell for that matter without an attack roll is not considered under the classification of 'Attacks'. So, no, magic missile is not considered making an attack anymore than Fireball is!!! Furthermore, if we were to determine what ascertains as a 'hit'.
Pg 194 Attack Rolls - When you make an attack, your attack roll
determines whether the attack hits or misses. To make an attack roll,
roll a d20 and add the appropriate modifiers. If the total of the roll
plus modifiers equals or exceeds the targets Armor Class (AC), the
attack hits.
So far Magic Missile can't be considered an 'Attack'. Since it doesn't require a d20 roll it's not considered an 'Attack Roll' under rules of page 194. Heck, it can't even technically land a hit based on these rules as you didn't meet or exceed any AC. Yes, the rules for Magic Missile say the darts hit a target but they don't land under rule terms what is considered a 'hit'. More on this at the end.
Finally, lets looks at the section of what happens when you roll a 1 or 20.
Pg 194 "Rolling a 1 or 20" - Sometimes fate blesses or curses a
combatant, causing the novice to hit and the veterans to miss. If the
d20 roll for an attack is a 20, the attack hits regardless of any
modifiers or the target's AC. In addition, the attack is a critical
hit, as explained later.....
Yes, there are rules to govern how to handle a Critical Hit. However, this is the only section in the book that describes how to acquire a Critical Hit. Again, you'll notice that it requires a d20 roll which Magic Missile does not. Since Magic Missile does not utilize a d20 we can't stretch the rules and say it auto-hits and even remotely try to call it a 'hit'. And since this is the only section that describes how to get a Critical Hit it is safe to assume the d20 roll is linked.
Finally, lets look at spell Saving Throws and Attack Rolls.
Page 204 Spells - "Saving throws" - Many spells specificy that a
target can make a saving throw to avoid some or all of a spell's
effects....
So making a saving or not only lets you apply the effects..again it doesn't count as a hit.This in addition to the d20 requirement is what protects players from being critted by Fireballs and Cone of Colds. Granted magic missile has no save so moving on to attack rolls.
Page 204 Spells - "Attack Rolls" - Some spells require the caster to
make an attack roll to determine whether the spell effect hits the
intended target.....
Remember that Attack Rolls all require a d20 roll. As such, your Magic Missile spell doesn't fall under this category. Even still, this only determine whether or the not spell's effect's hit. So yes spells that utilize 'Attack Rolls' can land 'hits' but Magic Missile does not.
Q.E.D Since no section in the book other Pg 194 Attack Rolls governs how one can acquire a critical hit, it is safe to assume that the only way to obtain critical hits is through Attack Rolls. Magic Missile does is not considered nor does it utilize an attack roll since no d20 is involved in the success/failure of the spell, thus it cannot land 'hits'. Since it doesn't land a 'hit', Magic Missile cannot crit via Rogue's Assassinate.
