Good Morning!
I'm going to chime in support for a weak battery or failed starter too: specifically because this is EXACTLY what my force outboard was doing two weeks ago.
Unfortunately my fix took a bit of time and considerable money: New Battery 85$, new starter $113, new plugs $11, and new Solenoid $48
Another posts suggests removing the plugs and trying, again another good idea; if the starter rotates the flywheel without plugs, it either not getting enough juice, or the starter is failed/failing.
I also had a bad ground in my console circuit board causing me electrical inconsitency at the key; so for real troublshooting you'll want bypass all that and jump straight to the starter (as suggested)
Good luck, I also agree that the flywheel is NOT locked.I had originally thought that, but I listened to the people posting here, and it turned out the suggestions were quite correct.
I have an excellent battery attached now too and it's cranking like a champ.
Compression test is fairly easy to do as well a decent gauge for the process can be bought at any hardware store for under $50
all the best!
-Pantz