[KevinH]

Why are you trying to create a checksum? Are you trying to validate your download?

If so, place the Sigil.app-1.9.10-Mac.txz file on your Desktop, then open Terminal.app and type the following:

cd
cd Desktop
shasum -a 256 ./Sigil.app-1.9.10-Mac.txz


The result should exactly match what is written in the Checksums file available on the releases page.

If it matches, your download is correct.

If after unpacking the app, it will not open, then right click on it and select open from the pop-up menu. Then immediately right click on it again, and select open again and this time Sigil will launch.

You only have to go through that once, the your Mac should remember.

This is all explained in that issue whose link was posted for you.