The manual does not indicate how to solve this problem.
Tighten garage door belt.
But basically assuming the belt is attached with enough slack removed the tensioner bolt should be able to taske up the rest of the slack once tightened.
Loosen the adjuster three turns counterclockwise if the door raised without.
1 close the garage door.
Step 1 closing the garage door before beginning use the garage door opener to close the door and fasten it.
Tighten the adjuster three full turns clockwise to add tension if the door closed and did not remain open halfway.
Brand new chamberlain garage door opener with excessive slack in belt drive.
The belt needs to be connected with the tensioner loosened so that the tensioner can be used to take up any excess slack.
This is a very simple job to do yourself in fact all you need to do is follow this simple 6 step guide.
The specific procedure should be in the manual for your unit.