I have never heard him make that claim.
Oh, I see your problem. You trusted dishonest sources that did not tell the whole story. Beau Biden served in Iraq:
"According to his father, Beau was diagnosed with
ankylosing spondylitis in 2001 after returning from service in
Kosovo.
[48] He was later diagnosed with brain cancer, which his father believes was possibly a consequence of exposure to military
burn pits in Iraq.
["
en.wikipedia.org
Could Biden be wrong about what caused Beau's cancer? Yes, he could easily be wrong. Does that make his claim a lie? Not even close.