Set your scene’s Preview and/or Frame Range to a multiple of your loop’s length for seamless playback

Redo Panel
Start Frame
Start frame for your animation loop.
Loop Length
Number of frames for your animation loop. Range to Loop assumes the last frame matches the first frame, so your range will end one frame before the last frame.
Loop Count
Sets the number of loops for an animation.
Preview Range
The Preview Range is a temporary frame range used for previewing a smaller part of the full range. When this setting is enabled, Range to Loop will set the Preview Range
Scene Range
The Scene Range is the full frame range of your animation. When this setting is enabled, Range to Loop will set the Scene Range.