How do I reinstall macOS 10.11.6 on old MacBook?

Sell it in its present condition.

There is no compelling need to install an operating system, besides, anyone interested in a Mac that old will either have a plan to install one (restore a TM backup, for example), or they won't care since that Mac is a better candidate for Linux than any version of macOS. It's a hobbyist's toy, at best.

There is a key chord corresponding to booting Recovery that will restore that Mac's originally installed operating system (Lion, probably) but the partition containing it may have been erased. Read How to reinstall macOS - Apple Support, and draw your attention to the following:

On an Intel-based Mac:

• If you used Command-R to start up from the local Recovery system, you get the current version of the most recently installed macOS.
• If you used Option-Command-R to start up from Internet Recovery, you might get the latest macOS that is compatible with your Mac.
• If you used Shift-Option-Command-R to start up from Internet Recovery, you might get the macOS that came with your Mac, or the closest version still available.

If you are bound and determined to install High Sierra, read High Sierra “The Recovery Server Could Not Be Contacted" Error. FIXED 2024 - Apple Community. Beware that I have not tried it personally but there are plenty reports of success.

Lastly, even though that Mac probably shipped with Lion, I have reason to believe it can boot and run Snow Leopard, which runs counter to Apple's generally valid statement that we cannot install a version of macOS / OS X that predates the version the Mac shipped with. You (or its future owner) can find retail Snow Leopard Install DVDs on the used market. Caveat emptor of course.

Install High Sierra via the Recovery workaround. Then download and make a bootable El Capitan USB installer if you really want it.

Or Shift-Option-Command-R boot to the oldest Internet Recovery and update that with downloaded El Capitan installer.

You cannot restore El Capitan. Since you erased that Mac there remains nothing to restore. You also have no Time Machine backup, so there is nothing to restore from one. You can only reinstall its originally installed operating system, or the one closest to it that remains available, and upgrade it.