According to Kant, all human experience minimally involves the use of empirical concepts. We don’t have access to anything like the raw sense data posited by many early 20th century logical empiricists, and it would not be of much use if we did. In Kantian terms, this would be a form of intuition without concepts, which he famously characterized as necessarily blind, and unable to function on its own.
Foundationalism is the notion that there is certain knowledge that does not depend on any inference. This implies that it somehow comes to us ready-made. But for Kant, all use of empirical concepts involves a kind of synthesis that could not work without low-level inference, so this is impossible.
The idea that any knowledge could come to us ready-made involves what Kant called dogmatism. According to Kant, this should have no place in philosophy. Actual knowledge necessarily is a product of actual work, though some of that work is normally implicit or preconscious. (See also Kantian Discipline; Interpretation; Inferentialism vs Mentalism.)
It also seems to me that foundationalism is incompatible with the Kantian autonomy of reason.