OS upgrade issues

Thanks for posting the screenshots, in your opening post you say you have a 2009 Mac Pro, from your About This Mac you have a 2009 MacBook Pro, which is a totally different machine, and cannot be upgraded using that tool, it is for Mac Pros only.

It does help matters if you post the correct details. This is what a 2009 Mac Pro looks like.

If you are still interested in using the latest iMovie on your MacBook Pro you will have to search using words like unsupported, High Sierra, macrumors.

If your Mac Pro 2009 is the 4.1 model then you cannot upgrade the OS further than El Capitan 10.11.6

So officially your machine is not supported, but there is a firmware upgrade that can be done so your Mac Pro will be recognised as a 2010 Mac Pro 5.1 which will allow you to upgrade your OS and therefore run the latest version of iMovie. Read this, http://forum.netkas.org/index.php/topic,852.0.html

Have you read the link I sent you and downloaded the Firmware upgrade tool, just follow the on screen instructions.

And does it not allow you to proceed any further.

Show me a screenshot of About This Mac please.

You may have to refer to the forum I linked to earlier to resolve your issue.