The Reagan administration took a conciliatory tone yesterday in discussing failure of its first formal effort to talk with representatives of Salvadoran leftist guerrillas, and chose to treat cancellation of such talks over the weekend as the result of crossed signals.

Both sides complained privately that the other had tried to impose conditions on the discussions, but each publicly reiterated willingness to try again.

Special presidential envoy Richard B. Stone briefed Secretary of State George P. Shultz on his truncated trip to El Salvador and Costa Rica. It began Thursday and was to include discussion in San Jose with the rebels about ways to bring them into the Salvadoran political process.

The trip ended abruptly Sunday after cancellation of the scheduled meeting.

"We have undertaken this effort with our eyes open, aware of both the potential obstacles and limitations involved, but in good faith," State Department spokesman Alan Romberg told reporters. "We will continue our policy of not imposing advance requirements on the holding of any meeting."

He refused to discuss the weekend's events, saying Stone's work "is best implemented with a large measure of confidentiality." White House spokesman Larry Speakes reiterated the comments later.

In a statement from Costa Rica, the combined guerrilla leadership of the Democratic Revolutionary Front (FDR) and the umbrella Farabundo Marti National Liberation Front (FMLN) said the groups remain ready to meet with Stone "on the basis of an open agenda and within a framework of agreement." They asked Costa Rican President Luis Alberto Monge to continue trying to arrange the meeting.

According to a State Department source, they failed because the guerrillas "didn't get their act together." There has long been internal dissension among the five groups in the FMLN about the value of any talks, and the possibility of talks apparently caught them with unresolved differences over a negotiating position, the source said.

Another State Department official added, however, that the guerrillas "had plenty of time to get organized if they had really planned to." Preliminaries to the attempted meeting have been going on at least since Stone's first trip to the region shortly after his appointment last month.

A third State Department official said FDR political leaders had said nothing new through the Costa Ricans, indicating that the left had sought agreement on an agenda as the first business. The guerrillas have called for a six-point agenda, including elections, power sharing and treatment of guerrilla and government armies.

On Sunday, the FDR/FMLN said in a communique that the talks had not been held because "it was not possible to obtain an agreement on procedural aspects." Asked about reports that the guerrillas had broken off the effort, Washington FDR/FMLN spokesman Alberto Arene said, "All speculation along the lines that we refused to talk to Stone, or that we are divided in our position, is totally false." He declined to elaborate.

The administration and the Salvadoran government have repeatedly said their goal is to have Stone "facilitate" talks between guerrillas and the Salvadoran peace commission, which is authorized only to discuss terms upon which the left might participate in planned elections later this year.

The guerrillas have called for U.S. participation in the talks and have said Stone would be acceptable as a delegate, not a mediator.

Stone is expected to brief congressional committees on his trip in closed sessions this week. State Department officials refused to speculate on the next step in getting the talks started. "We're going to wait a bit and see what happens," one said.

In a related matter, the Organization of American States announced that its permanent council will begin a special debate on Central America here Thursday.

The special session was requested by Roberto Martinez Ordonez, Honduras' OAS ambassador, "to discuss the grave events and circumstances that characterize the critical situation" in the region, OAS spokesman Dan Cento said.

The 30 OAS ambassadors on the permanent council are expected to resume discussing a Honduran proposal for regional negotiations among all five Central American countries, he said. That was tabled without a vote in April.

After several days of debate at that time, the council was unable to agree after Nicaragua objected that any talks must include the United States as a central participant in regional issues.