What causes failure to boot into recovery mode

MacPro mid 2012, 10.13.6

I need to re-install the OS due to minor corruption caused by a failing system drive. I've replaced the drive by cloning the original drive. It boots normally just fine but a few programs (Preview) won't run due to corrupt files. When I try to boot into recovery mode, I briefly get the circle with a line through it then it reverts to starting normally. What might be causing that failure and/or how can I get a 10.13.6 system installer?

Thanks!

Bob Smith