WHY WILL MY EARLY 2009 MAC PRO 2.66 GHz Quad-Core Intel Xeon NOT UPGRADE TO SIERRA

Your signature says you are running 10.6.4, Snow Leopard. If this is the case you need to update this to 10.6.8 in order to get the Mac App Store:

Mac OS X 10.6.8 Update Combo v1.1

Then you have to upgrade to El Capitan before you can upgrade to Sierra:

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12

Upgrade the firmware of your MacPro4.1 to that of a 5.1 and then Sierra will install normally

http://arstechnica.com/apple/2011/05/firmware-hack-can-transform-a-2009-mac-pro- into-a-12-core-monster/

According to Apple if he is running Snow Leopard he still needs to install El Capitan before he can install Sierra. I'm assuming his signature is accurate, of course.

That is likely true in this case, but I am not sure that is strictly true in every case. To UPGRADE to Sierra, you need to be running 10.11.5. But it appears you can "Buy" it from any other Mac OS 10.7 or later (except 10.6.8 is not listed -- possibly because 10.6.8 REQUIRES a Mac App Store Update before the Mac App Store is fully functional.

So I agree the OP needs to install SOMETHING higher that the 10.6 they are running now, but in certain cases, that could be an intermediate version, if available to them.

How to get macOS Sierra - Apple Support

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