Clone your 10.6.8 system to the drive and then boot from that drive. You should be able to get Sierra to install from App Store but I don't know if you can still get El Capitan. Do you have the latest version of 10.6.8? If you don't, that could account for the inability to access App Store (unless Apple has temporarily shut it down, which happens occasionally).
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
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