The Symantec Endpoint Protection (SEP) Small Business (SBE) client fails to install because another install is already in progress. You may see a pop up as the install is progressing that the install cannot complete.
In the C:\ProgramData\Symantec.cloud\syminstall\SymantecLauncher.log
2017-03-10 16:11:26.111; INF; t=4964; CMsiExecuteMutexChecker::IsInProgressRegKey(126) : The SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\InProgress key is detected, another MSI process is running.
In task manager you will see another install process already running.
Another MSI install is currently running.
Wait until the MSI install which was taking place prior to installing SEP SBE to finish before attempting the SEP SBE install again.