Symantec Management Agent install on new computer fails after upgrade to 7.6 or 8.0 build if "Override the default installation path:" was provided in "Symantec Management Agent Installation Options" earlier.
For Push install, the SMP Console will show Install completed successfully but the Agent is not installed and the error can be seen in Agent Install log (AEXNSC.log) on client.
In Pull install the same error appears in GUI.

If run manually AeXNSC.exe (or AeXNSChttp.exe or AeXNSCHttps.exe), you can see the following popup window message:

Install failed. Error message: Failed to run action: CopyAgentFilesTree

The AEXNSC.log (under C:\ProgramData\Symantec\Symantec Agent\InstallLogs), you can notice the following entries referencing the nonexistent G:\ Drive:

2018-02-16 11:33:10 P:744    T:5868   *ERRR CoreUtil: GetInstallPath could not get the install path from InstallDir - error: 0x2
...
2018-02-16 11:33:10 P:744    T:5868    INFO Installer: %AGENT_INSTALLATION_FOLDER% => G:\program files\Altiris\Altiris Agent
...
2018-02-16 11:33:10 P:744    T:5868    INFO Installer: %PLUGINS_INSTALLATION_FOLDER% => G:\program files\Altiris\Altiris Agent\Agents

ITMS 7.6, 8.x

Change in GUI and options for Symantec Management Agent Install Settings page from ITMS 7.6 onwards

1. Click on Default settings for Symantec Management Agent Install policy on Symantec Management Console> Settings> Agents/Plug-ins> Symantec Management Agent >Settings> Symantec Management Agent Install 
2. Clear the path provided in "Override the default installation path:" under Agent Install Settings tab
3. Click on OK to update the settings

Now the Agent will install successfully with both push/pull method.