There are two types of agnosticism, one of which is inherently self-contradictory.
Here, we will examine why the first position, that "Truth cannot be known," is an impossibly self-contradictory position.
In short, "truth cannot be known" is a self-contradictory position because if truth cannot be known, then the supposed "truth," "truth cannot be known," cannot itself be known.
If it is true that "truth cannot be known," then it is also true that "truth" can be known, since "truth" that "truth cannot be known" can be known. However, if truth both can and can't be known at the same time, then truth is self-contradictory, which is impossible.
If truth both can and can't be known at the same time, then any statement could also be both true and untrue at the same time, which is absurd. No statement at all would have any meaning whatsoever.
The necessary conclusion to all of this is that the position, "Truth cannot be known," cannot be true because it is impossibly self-contradictory.