First go ahead click Planner and then Posts. Then go ahead and click Create new

Choose your Instagram account and upload a video for your Instagram Reels

Go ahead and click the little Instagram Icon and then choose Post as a Reel as shown in the picture. Once that's done you can go ahead and click Schedule for later which is located on your bottom right corner

Choose a date and then click Done

Double check the post and then go ahead and click Schedule

Your post should be scheduled as shown in the picture