I’m trying to utilise an older iMac that we have in a corner for my Zwift workout, its running OSX High Sierra, an iMac 1715.
The program starts and then a fault window appears, the header is below
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000068
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: exc handler 
Is this fixable please?
High Sierra should be okay. Is the system fully patched? Also, are you running the latest version of Zwift. It should auto-update when you launch. But if it’s crashing, you might want to put Zwift in the trash. Also trash the Zwift folder in either Downloads or Documents (I forget which). Empty your trash. Then download and install again. When installing it should ask for permissions, make sure you say yes/okay.
I think I’ve done all of that
Any other suggestions?
I’m not sure exactly which machine you have, but High Sierra hasn’t been updated since November 2020, which exposes you to security vulnerabilities and other bugs fixed in newer versions of macOS. If you have the option to install a version of macOS with current Apple support, I recommend it. It might also help you run newer applications like Zwift (but I don’t know if it will help). If your hardware is no longer supported by Apple, you may be able to use OpenCore Legacy Patcher to install a current version of macOS. You can find tutorials for this on YouTube.
Error message doesn’t tell me much.
Guessing the iMac works fine for everyday tasks, e.g, internet browsing, music, etc…
What year iMac is it? Bluetooth 4.0 which is required for Zwift wasn’t until 2012 if I remember correctly. You can check Bluetooth version via Apple > About this Mac > System Report > Hardware > Bluetooth