The Labour manifesto accepted the result of the referendum. It said quite clearly "Labour accepts the referendum result".
"We will scrap the Conservatives’ Brexit White Paper and replace it with fresh negotiating priorities that have a strong emphasis on retaining the benefits of the Single Market & Customs Union"
"Labour recognises that leaving the EU with ‘no deal’ is the worst possible deal for Britain"
Which is why Labour backed the EU Withdrawal (No2) Act (aka the Benn Act)
1. Try to amend their deal (which we did in Govt talks)
2. Back a public vote so the people have the final say
And that's what subsequent Labour conferences backed