High Sierra install I get the NO symbol

Trying to install HS on my iMac 27-inch, late 2015. After downloading and rebooting, it immediately goes to black screen a the no symbol (circle with line through it). after a minute it looks like its trying to boot into windows (series of dots moving in a circle).

I've restarted in safe mode, ran disk utility, no errors.

re-installed Sierra.

Tried to erase the drive, and re-install Sierra, then tied to install HS. Does same thing

Wiped the drive, removed the Boot Camp partition. re-installed Sierra, tried to install HS, same results.

Partitioned one of my external drives, installed Sierra on it, then downloaded and tried to in stall HS. Same results.

I'm running out of things to try. Any suggestions?