Tap/hold on a space until it selects, then slide your finger* (down, across etc.) to complete the selection;
or after the space selects, drag the blue selection handles to set your loop.

Either tap the arrows in the first selected measure or the Play button to begin loop play.

 

 

* External keyboard: Click/hold on a space until it selects, then drag down, across (etc.) to complete the selection; or after the space selects, drag the blue selection handles to set your loop.

Either click the arrows in the first selected measure or the Play button to begin loop play or press the Spacebar.