So I downloaded the SMAPI 2.6 installer zip not the developers one as I m not a developer and followed instructions that I've used with every SMAPI update by opening my Terminal, typing 'mono', putting a space then dragging the Install on linux.sh into the terminal and hitting enter, did not work.

Tried to do the same with the install on Mac.command, did not work. got this same message in my terminal with both those attempts at updating SMAPI the regular way-

Cannot open assembly '/Users/myname/Desktop/SMAPI 2.6 installer/install on Linux.sh': File does not contain a valid CIL image.

note to keep my privacy i replaced my actual name that was there with myname it obviously did not show it as myname in the terminal

This is the SMAPI crash log in case that was needed as well- https://log.smapi.io/48qr5XiQ

You don't need to install through the terminal like that anymore, just run install on Mac.command instead (double-click the file and choose 'run' or 'execute'). See the install guide for more info.