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Entangle (PHB p.238) states that vines sprout from the ground in a 20-foot square:
Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range. For the duration, these plants turn the ground in the area into difficult terrain.
Is it possible for a druid who is falling (under the effect of feather fall) to cast entangle on a wall in a square perpendicular to the wall (rather than along the wall) in such a way that the vines reach 20 feet from the wall?
Grasping weeds and vines sprout from the ground in a 20-foot square starting from a point within range.
The rules do not define what the word ground means, which means we use the common English definition:
the solid surface of the earth.
Thus, according to the rules as written and common English definition, a traditional manufactured wall is clearly not ground.
As Jeremy Crawford has said:
Saying that a wall is the ground defies the idiomatic meaning of the word "ground." But a DM is free to allow bonkers things.
Which leads me to my next point...
However, there are definitely edge cases here. For example when you are falling down a pit dug into the earth. Or alongside a cliff face. However, these are edge cases that the DM must decide since they can go either way and the rules are vague.
If your DM is willing to accept an even more lenient view of what ground is, they could even extend the definition to include things like walls and structures. However, this would be a houserule at your table. It also has the potential downside of making the spell potentially a lot more versatile than the designers intended. After all, if walls could count as ground potentially any solid surface could. If your DM is fine with this though, it is within their power to rule it as such.
As Jeremy Crawford has also said:
The DM decides how generously to interpret words like "ground." Unless we redefine or focus a word, we use it in its idiomatic English sense, knowing that some words are open to creative interpretation.
It is important to note that there are other spells that also must be cast on the ground so a DM should consider carefully before allowing it for all.
"Is it possible for a druid who is falling (under the effect of feather fall) to cast entangle on a wall..."
Your specific example certainly seems to be an edge case and as a GM, I would allow the vines to sprout from the wall surface as it would be considered "ground" in the sense plant life could certainly grow through it over time.
"...in such a way that the vines reach 20 feet from the wall?"
I would not, however, allow the vines to extend 20 feet from the wall. It is my assumption the spell is intended to be a large quantity of short vine lengths which erupt from the ground in the area. I don't think the intent is to create long strands from which to swing and certainly not sturdy branches extending straight out of the surface a great length of 20 feet.
Players close to the wall could use the entanglement as handholds, so this could possibly still save them as it is a very creative use of the spell.
I think the spell that would benefit more in this situation is Grasping Vine. Again, it uses the word Ground, but using GM powers we tend to allow that to occur anywhere that a vine would naturally grow, so earthen walls are ok.
You conjure a vine that sprouts from the ground in an unoccupied space of your choice that you can see within range. When you cast this spell, you can direct the vine to lash out at a creature within 30 feet of it that you can see. That creature must succeed on a Dexterity saving throw or be pulled 20 feet directly toward the vine. Until the spell ends, you can direct the vine to lash out at the same creature or another one as a bonus action on each of your turns.
Entangle would work at a push if the player was next to the wall, but 20 ft out is a little too far.
