Select the Time Remapping effect from the layer's composition panel to activate it, and from the Menu bar, select Animation > Add Expression.

Then double click on the last keyframe and delete it from your composition.

On the very last keyframe, double click on it and change the time to 0.

Move to the very last key frame, step backwards one frame, and create a new keyframe.

Time Remapping will create two key frames: one at the beginning of your composition and one at the end.

Now, in the new composition, go to the Menu bar and select Layer > Time > Enable Time Remapping.Then place the newly trimmed composition that you wish to loop (from the project panel) into your newly created composition by dragging it to the new composition panel (See note *). Create a new composition ( Command N) and keep your settings the same as the original composition.Then from the Menu bar, select Composition > Trim Composition to Work Area. Set your work area start and end points to the section of the composition you wish to loop.This will loop your selected composition.

After effects