For a visual on what AD is referring to, here is what the prop hub is about (
link) and here is what the motor coupler is about (
link). These are a representation but as you can see, hopefully it’s the prop hub that is the issue.
More: in looking closely at your pics I did not see an indication of a cotter pin in your prop nut keeper > this is the stamped metal device over the nut that is used to keep it from loosening > but it needs a cotter pin.
Assuming it is the prop hub, make sure you have all the needed components > they make kits like shown in the video to replace these items. Your local marina will be a great resource for the items needed. And follow the instructions for torquing the prop nut. If you do not have a torque wrench, buy one. Here is one that is inexpensive and will work
link. Get the proper size socket at the same time.
Also, you did not say if there was an event that caused this issue, an event like running aground with the prop. If so, checking the runout of the prop shaft and / or doing a pressure test on the sterndrive may be in order.
Continue to post any / all questions and help needed and good luck!