I erased the disk and am trying to reinstall it, but I keep getting this error.

The reason is that the OS is no longer supported, and the security certificate has expired. This is the error.

You need to open Terminal in recovery mode and set the date before the security certificate expires. The latest El Cap update, 10.11.6, was released on July 9, 2018.

Command: date [MM][DD][HH][MM][YY]

Yes, the format is strange because the time is inserted between the day and year.

This will set the date and time to July 10, 2018, at noon; The certificate hasn't expired on this date:

date 0710120018

Another requirement: disconnect from the internet, otherwise your computer may resynchronize with the time server.

I used: date 0101000016