PLEASE HELP! OSX cannot be installed on Macintosh HD

Question

PROBLEM
I'm trying to update from Snow Leopard 10.6.8 to Yosemite 10.10.2 and it's not working. I downloaded the installer file from the app store and it is in my applications folder. When prompted, I "Select the disc where I want to install OS X" and select Macintosh HD, causing a bubble to pop up saying "This disc cannot be used to start your computer". If I press the back button, then go back forward it simply says "OS X cannot be installed on Macintosh HD".
TROUBLESHOOTING ATTEMPTS
I have tried to repeat the process about ten times with the same results.
Reinstalling the installer doesn't help.
I ran Disc Utility, which claimed I had an error in the number of blocks in my volume, booted in Recovery Mode and repaired the disc. No change.
Other online threads told me to find and delete the file "backups.backupdb" on my HD, but I can't find that anywhere on my computer using the Finder Search function
SPECS
Late 2011 MacBook Pro
OS X 10.6.8
2.3 GHz Intel Core i5 processor
4 GB 1333 MHz DDR3 Memory
65GB available disc space

Answer 1

You must have a really fouled up Snow Leopard system. Please do this:
Repair the Hard Drive and Permissions
Boot from your Snow Leopard Installer disc. After the installer loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After DU loads select your hard drive entry (mfgr.'s ID and drive size) from the the left side list.  In the DU status area you will see an entry for the S.M.A.R.T. status of the hard drive.  If it does not say "Verified" then the hard drive is failing or failed. (SMART status is not reported on external Firewire or USB drives.) If the drive is "Verified" then select your OS X volume from the list on the left (sub-entry below the drive entry,) click on the First Aid tab, then click on the Repair Disk button. If DU reports any errors that have been fixed, then re-run Repair Disk until no errors are reported. If no errors are reported click on the Repair Permissions button. Wait until the operation completes, then quit DU and return to the installer.
If DU reports errors it cannot fix, then you will need Disk Warrior and/or Tech Tool Pro to repair the drive. If you don't have either of them or if neither of them can fix the drive, then you will need to reformat the drive and reinstall OS X.
If you continue having errors reported, then you should backup your files, erase the drive and reinstall Snow Leopard. Be sure to backup the Yosemite installer you downloaded if you don't want to download it again. Otherwise, try the installer again.