As an explanation, it's worth including the next few lines of the introduction as well:
What manner I mean, will be quite clear to the reader, who must soon perceive great inexperience, immaturity, and every error denoting a feverish attempt, rather than a deed accomplished. The two first books, and indeed the two last, I feel sensible are not of such completion as to warrant their passing the press; nor should they if I though a year's castigation would do them any good; - it will not: the foundations are too sandy.
In other words, Keats was under the impression that his work - that was being published here - was written with "inexperience, immaturity, and every error". He is saying that his own writing displays a lack of experience and maturity, and was full of mistakes.
He says that the finished work is more of a "feverish attempt, rather than a deed accomplished"; saying that it's not so much a completed work as a delusional, incomplete attempt to write something.
Keats continues to disparage his own writing, saying that they're not really good enough to be printed, and that even a year of editing wouldn't help, because the "foundations are too sandy" - the base of the work (again, his own writing) was too "sandy" - the metaphor being that it's not a firm enough base to improve on, since the original work wasn't of good enough quality.
It's basically a long-winded way of saying that, as is the case with every single writer on Earth, he doesn't think much of his own work. It's a lot of big words to use on being self-deprecating.