Does the amount of memory allocated, number of cores, and disk space allocation meet the minimums for R81.20? This is a common cause of problems in a VMWare environment, I use R81.20 in VMWare Workstation for training and it works fine with the proper resources allocated. The minimum is 2 cores, 8GB RAM and 110 GB disk space for an SMS but I'd strongly recommend at least 4 cores, 16GB of RAM, and 250GB of disk space.
Also set the Guest VM version to "RHEL 7 64-bit" to ensure it creates the proper and most efficient disk/network devices for you.
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com