My 12000 says this every time I turn it on:  "Raise Presser Foot and Needle Bar" then turn power on again.  I have started to wonder if there is something amiss when I turn the machine off?  The only way I get the message to go away, is to cycle the Needle Bar by hand a complete cycle.    The shop where I bought this lovely machine is about 6 weeks out in service appointments.    Pardon me if this has been discussed--I tried searching but didn't find a previous discussion.  Thanks for any tips.

The sensor that detects the needle at the top may be out of adjustment. This is a simple adjustment, but requires a technician. Maybe you can make an appointment so you can keep using it until they can see it.

I have MC 9900 and after using the machine for several years I started getting this message about 2 years ago. It seems to occur every time I start my machine having not used it for a few months. It was once suggested to me to slide a folded piece of paper through the right side of the thread guide. This worked the second time the message came up and I was able to sew for several months without incidence. I have not sew for 4 months and message has returned and this technique did not clear the message.

There has got to be a way I can adjust this myself. When I took it in for repair the first time it cost me $150 and that is too much for me to have to spend to correct this every time I use my machine after a break in sewing.

Is there some way to purchase or download a maintenance manual?

You can download a manual here: https://www.manualslib.com/manual/1241003/Janome-Mc9900.html
The message indicates that the power-on self-test has failed. There are several different sensors involved, and the message gives no indication of which one failed. The manual will give you access to the internal diagnostics, but that may be difficult to use, as they sometimes require special tools or gauges. One thing you could do is lay the machine on its back and remove the bottom. Most of the machines have a bottom cover that’s like a tray, held on with 4 screws at the 4 corners. There will likely be a lot of lint and dirt. You can vacuum it out, but be careful not to dislodge any springs or other parts.

Did a lot of sewing yesterday and today, got same message multiple times, seems I have better luck when I leave presser foot down (as we used to do for vintage machines) and cycle the needle up and down one complete cycle by hand, before shutting off power.  I suspect there may be a sensor, as you suggested.  Will also try the sheet of paper along the right side of thread guide as Motchka suggested.

FWIW, I went to that "manualslib.com" site to check for the 12000 manual, and my computer lit up like an Xmas tree warning me of viruses....YMMV.

The safest thing is to go to the site from an iPad. Windows malware cannot infect an iPad, though you may get fake “Your computer is infected” messages if it tries.