Added R Services to SQL Server 2016 Enterprise - Cannot get past Msg 39021
I know it is quite a while, but I had the very same error yesterday after 
adding SQL R system Services to an SQL2016.

Besides the need to manually 


add / grant  "NT Server\MSSQLFDLauncher" "LogonAsService"  (secpol.msc, or create a approriate Group)
add / grant "\SQLRusergroup"  "Allow logon locally" (secpol.msc)
to make SQLServer laucher start at all,


it turned out in my environment that the long name (I had something very similar to 
"M:\SQLData\MSSQL13.MSSQLSERVER\MSSQL\ExtensibilityData") was causing a fuss - 
despite it did not include a blank as described in some findings in WWW concerning this.

I did the following:


created a short folder name "E:\MSSQL_R" 
xcopied the directories from e.g. E:\SQLData\MSSQL13.MSSQLSERVER\MSSQL\ExtensibilityData there (/s/e/v to include empty dirs)
Gave NT Service\MSSQLFDLaucher "FullControl" NTFS rights to this
changed the "WORKING_DIRECTORY" path in rlauncher.config to point there


et voila - test query worked after that.