“To be quite clear, no government can go to a parliament and say: here is our proposal, can you guarantee a result?” Schulz told reporters after the meeting with Cameron.
"We are concerned about some elements [of the UK deal]," he said. The deal, proposed by Cameron, is set to be discussed by European leaders at a two day summit in Brussels starting on Thursday.
However, Schulz said the meeting with Cameron had been "very constructive" and that he would work to pass legislation agreed by EU leaders at the summit.
But he also warned the legislative process can be unpredictable. Such political operations are always "fragile", and there is a need to compromise between member states, institutions and the European parliament, he said - but the European parliament does not have a veto.
Speaking to Sky News, he rejected the idea that MEPs have a veto, saying they will work within "a frame" that the EU summit will set.
Earlier this month, draft proposals were backed by a number of national leaders, but Cameron needs to win over the European parliament, which must approve the central elements of any deal, including restricting benefits.
Meanwhile Jean-Claude Juncker, president of the European Commission, has said there is "no plan B" when it comes to Britain leaving the EU.
Schulz encouraged British people to vote on the outcome of Thursday's negotiations, which he hopes will be a vote to remain part of the 28-member bloc.
Cameron is today set to meet a number of senior MEPs, as well as European Commission President Jean-Claude Juncker to discuss the renegotiations.