We blew it.
The 'search for America' by Wyatt and his side kick Billy is not geographical, it is a literal quest to find out where America's head is at.
When Wyatt exclaims "we blew it" he is not talking about their personal journey, but rather referring to America as a whole. Wyatt and Billy had left LA looking for America and freedom. What was found was unexpected and disappointing. The beginning of their journey was promising - the farm and the commune both shared great potential for personal independence and strong, tight knit communities. Both placed an emphasis on living off of the land, which was an important aspect of American life on the frontier. But as Wyatt and Billy travel further east they are met with persecution for their long hair and unruly looks. They (and George) are victims of hate crimes and ultimately murder.
Wyatt is making the point that we -America as a whole- blew it. There was potential for freedom and independence but somewhere along the way prejudice and intolerance won out.