I installed Mac OS X Lion 10.7.5 on an ASUS P8Z77-V LE motherboard with an Intel Core i7-3770K processor using Unibeast/Multibeast. I wanted to upgrade it to Mac OS Sierra, but the App Store says it can't complete the purchase since it can't be installed on this machine, and the more details was about what kind of machine Lion needed to be installed on, rather than Mac OS Sierra.
Here is what my System Information says:
Model Name: iMac
Model Identifier: iMac12,1
Processor Name: Intel Core i7
Processor Speed: 3.51 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Memory: 8 GB
Boot ROM Version: IM121.0047.B00
SMC Version (system): 1.30f3
Serial Number (system): SOMESRLNMBR
Hardware UUID: B1E22D47-CB68-557F-8955-B9CD93CCA08C
What's wrong? I believe the system fits the requirements for Mac OS Sierra. I even tried downloading El Capitan, but the App Store gave me the same response.
EDIT: OK, I'm a Customac noob. Looking closer I noticed the serial number was bogus. It appears I need to address that. Not sure why Multibeast didn't handle it. Probably because I didn't choose either Easy Beast or the DSDT "do everything" options, since those didn't work for me. Instead I went through and only chose what was necessary. Including installing Chimera, but I guess the serial number generation doesn't happen in that case.