United Nations Ambassador Andrew Young said today that President Carter cannot afford the strategic arms limitation treaty if winning Senate approval means cutting domestic spending to boost the defense budget.

Young made the remark in a news conference after a speech to black dentists.

Sen. Sam Nunn (D-Ga.), a key undecided vote on the SALT II agreement, last week said he could not support the arms pact unless Carter agreed to a substantial increase in Pentagon spending for conventional weapons.

Young said if that means a cut in social welfare programs, Carter would have to sacrifice SALT II.

"I don't think he can pay that price and keep his other commitments," the former Georgia congressman said. "I think it would be better that we don't have a SALT treaty.

"I think President Carter's primary commitment is to a balanced budget and ending inflation. A 5 percent increase in military spending is not going to help him reach those goals; it's not going to help provide much for national security.

"I think that our national security right now is much more jeopardized by the economic situation we find ourselves in than by our military situation."

Asked about criticism of the Carter administration's performance of 1976 campaign promises to blacks. Young said black Mayors Coleman Young of Detroit, Maynard Jackson of Atlanta and Tom Bradley of Los Angeles and the congressional black caucus have not rebuked the president publicly. He shrugged off the criticism of National Urban League President Vernon Jordan, noting that the league never endorses candidates.

"I think there's a distinction between criticism of a president because of high expectations and rejection of a presidency," Young said. "I have not sensed in the black community a rejection of the president."

Young said Carter has "done a good job" appointing blacks to judgeships and regulatory boards, but "he's running against a concept of perfection."

At the same time, he added, "There is no Republican alternative" for blacks in 1980.