Web 8.5 installation failed during Topology Manager install with the below error:

Error occurred while attempting to communicate with Topology Manager Service at 'http://localhost:81/ttm201601'.  The server returned the following response code: 500 (InternalServerError)

1. Go into IIS and click on the top node > select Feature Delegation under the Management node. 
2. Enable ISAPI Filters and Handler Mappings to Read/Write.
3. Restart IIS for the setting to take effect:

The ISAPI Filters and Handler Mappings IIS need to be set to Read/Write permission

To be sure the issue is caused by the ISAPI Filters and Handler Mappings  permission, there are two clues to look for.

1. In the Translation Manager log, this error is seen:

CreateCMEntity: Run PS command: Add-TtmCmEnvironment 
 -Id 'W12R215' -CoreServiceRootUrl 'net.tcp://localhost:2660' 
 -WebsiteRootUrl 'http://W12R215.USTridionCS.com:80' 
 -AuthenticationType 'Windows' 
 -UserName 'W12R215\MTSUser' 
 -Password '****' 
 -TtmServiceUrl 'http://localhost:81/ttm201601'.
CreateCMEntity: error Error occurred while attempting to communicate with Topology Manager Service at 'http://localhost:81/ttm201601'. 
 The server returned the following response code: 500 (InternalServerError).
MSI (s) (F0!FC) [06:07:45:900]: Product: SDL Web Topology Manager -- Error occurred while attempting to communicate with Topology Manager Service at 'http://localhost:81/ttm201601'. 
 The server returned the following response code: 500 (InternalServerError).
Error occurred while attempting to communicate with Topology Manager Service at 'http://localhost:81/ttm201601'. 
The server returned the following response code: 500 (InternalServerError).
CreateCMEntity: Delete user account _Tridion_Temp_User_.
CustomAction CreateCMEntityExecute returned actual error code 1603 
 MSI (s) (F0:18) [06:07:59:865]: Note: 1: 2265 2:  3: -2147287035

NOTE:: The MSI install error code -2147287035 means access denied as explained here: https://support.symantec.com/en_US/article.tech12638.html


2. The next clue is in the IIS log..
If you see the below log message in the IIS log for the Content Manager website...

2017-09-28 13:23:23 127.0.0.1 POST /WebUI/Models/TCM54/Services/General.svc/GetUserSettings - 80 - 127.0.0.1 Apache-HttpClient/4.5.2+(Java/1.8.0_121) - 500 19 33 5

NOTE: The http subcode is 19, which means Configuration is invalid