The Tarrasque would be pushed to one side of the wall when you cast the spell:
If the wall cuts through a creature’s space when it appears, the creature is pushed to one side of the wall (your choice which side).
If you cast the wall inside the beast, it clearly "cuts through a creature’s space when it appears", and therefore "the creature is pushed to one side of the wall".
Of course, at that point it wouldn't hinder the creature's movement much.
Regarding how this should be narrated by the DM, there isn't much to go on in the spell's description.
For example, what if the wizard shapes the wall into a sphere around himself? This is such an unusual situation, so the DM would have quite a bit of latitude in determining how the result is narrated.
It is fun to imagine that the wizard, in a giant bile filled snow globe, is unceremoniously ejected from the Tarrasque like a hairball from a cat. I'm not sure who comes out ahead in that scenario.
A wall of force grants cover by being an obstacle. A confirmed tweet from a game designer states this includes spells.
According to the cover rules in the Player's Handbook, p.196:
Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. [...] A target has half cover if an obstacle blocks at least half of its body [...] A target has has three-quarters cover if about three quarters of it is covered by an obstacle [...] A target has total cover if it is completely concealed by an obstacle.
Is a wall of force an obstacle? In D&D 5th edition, words not defined in game rules are interpreted according to their standard English meaning.
Something that impedes, stands in the way of, or holds up progress.
The description of the wall of force states:
Nothing can physically pass through the wall.
Therefore, the wall of force is an obstacle, and being behind it relative to your opponent grants you cover.
If I understand your recent questions correctly, you hope to learn whether wall of force specifically blocks magic, on the hypothesis that magic is not "physical". The rule on casting spells against people behind obstacles appears in the Player's Handbook, page 204:
To target something, you must have a clear path to it, so it can't be behind total cover.
D&D 5e designer Jeremy Crawford, whose rulings are considered official, confirms in a tweet that wall of force provides total cover:
Q: could a wizard make a sphere around a creature using wall of force and then chill touch to damage them through the wall?
Crawford: Unless a spell says otherwise, you can't target someone behind total cover (PH, 204)
Also here, in specific reference to wall of force:
Cover is a physical obstruction, not necessarily a visual one.
While the cover rules say that total cover "completely concealed", a term which in earlier editions of D&D referred specifically to visibility and not cover, Crawford here appears to clarify that "concealed" is a synonym here for "covered". Note how the other two forms of cover say "blocks" and "covered", so it seems that they're using synonyms to avoid repetition.
Therefore, regardless of the "physicality" of magic, the official ruling on this issue is that wall of force provides total cover, and therefore blocks magic.
Best Answer
Mike Mearls’ unofficial ruling is that Wall of Force does block spells, including lines of effect
Quoting Mike Mearls on Twitter:
Rulings from developers on Twitter are in no way considered official rulings, but this is still a ruling from someone who knows the game supremely well.