what is the latest operating system I can download for this old imac?

Back up your data, check your applications for compatibility, click here, and then on the download link for High Sierra, Mojave, or Catalina.

Catalina and newer won't run 32-bit software; download and use Go64 to scan for it. To look other programs up, click here.

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A Late 2013 iMac can run up to macOS 10.15 Catalina.

see > How to get old versions of macOS - Apple Support

You have attempted to install an incremental OS update (10.13.2). You need to use the links provided by Niel or den.thed above. Those links will provide the appropriate OS file.