[RPG] What happens if you Teleport into a wall

You could use the existing "false destination" option

The Teleport spell describes probabilities of what happens based on your familiarity with the destination. One of the options is "false destination":

"False destination" is a place that doesn't exist. Perhaps you tried to scry an enemy's sanctum but instead viewed an illusion, or you are attempting to teleport to a familiar location that no longer exists.

Attempting to teleport into a wall isn't precisely the same as attempting to teleport to a nonexistent location, but I believe the idea is similar enough. Certainly, if you intentionally tried to teleport to a room on the other side of a wall, but it turns out that behind the wall is just solid stone rather than a room, that would fall under "false destination". In this case, you're not intentionally teleporting into a wall, but rather being sent there due to an off-target teleport, but otherwise the situation is similar. So, in absence of any other rules about what happens in this situation, it's reasonable to decide by rolling on the "false destination" table. Essentially, I'm extending "False destination" to also encompass "Invalid destination". (Note that you could also extend this to other "invalid" destinations, such as a location that is warded against teleportation by a Forbiddance spell.)

Note that when you roll a "similar area" result, you should probably choose an area similar to the intended destination, as opposed to just choosing another similar wall/rock to teleport them into. If you use this to resolve teleporting off-target into a wall, the end result will be that the caster ends up maybe taking one or more rounds of "mishap" damage before being dropped in an area near the off-target location that is similar to the intended destination. You can describe any resulting force damage from mishap rolls as damage that they take due to the spell trying and failing to teleport them into the wall before redirecting somewhere else.