Hi there. I'm no expert, but does your iMac support AVX2? If not, then it might not be able to run the game at this time. If you need to find out, open Terminal and type:
sysctl -a | grep machdep.cpu.features
Near the end of the line you'll see "AVX1.0" or something else, e.g.
machdep.cpu.features: FPU VME DE PSE TSC MSR PAE MCE CX8 APIC SEP MTRR PGE MCA CMOV PAT PSE36 CLFSH DS ACPI MMX FXSR SSE SSE2 SS HTT TM PBE SSE3 PCLMULQDQ DTES64 MON DSCPL VMX EST TM2 SSSE3 CX16 TPR PDCM SSE4.1 SSE4.2 x2APIC POPCNT AES PCID XSAVE OSXSAVE TSCTMR AVX1.0 RDRAND F16C
