Former England captain Beckham, 39, had two plans for a 25,000-seat stadium in central Miami rejected last summer.
MLS will consider a report from the league's expansion committee this weekend and Garber indicated some decisions would be made early in 2015.
"This can't go on forever," Garber said of the delays with Beckham's franchise.
Ex-Manchester United midfielder Beckham, who hinted in the summer he may come out of retirement to play for his un-named franchise, has been urged to ground share with an American football team.
Garber says the league is not performing as hoped financially and its franchises are combining to lose more than £64m ($100 million) annually.
He added that a Miami-based side remains an objective for MLS if the ground issue can be solved.
"The Beckham group continues to work to try and find a stadium solution that they believe will put them in a position to succeed," Garber said.
The league will grow to 20 teams in 2015 with the addition of New York City FC and Orlando City FC followed by Atlanta and a second LA-based team in 2017.
Talks have also been held with groups in Minnesota, Sacramento and Las Vegas.