Couldn't create symbolic link for Extensibility Data

Question

I am trying to set up SQL R Services in an instance of SQL Server Express.

I am getting the error

Unable to launch runtime for 'R' script

and have tried to implement the solution found here.

The following command works successfully in an elevated command prompt:

"X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\R_SERVICES\library\RevoScaleR\rxLibs\x64\RegisterRExt" /uninstall

However, the subsequent command fails:

"X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\R_SERVICES\library\RevoScaleR\rxLibs\x64\RegisterRExt" /install

With the following error message:

> Source directory to pick the RExtension binaries determined to be "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\R_SERVICES\library\RevoScaleR\rxLibs\x64\".  
> Installation flavor was not supplied  
> Connecting to SQL Server...  
> SQL Server binn directory is "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\Binn".  
> Creating user account pool...  
> Working directory from data root "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL" is "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\ExtensibilityData".  
> Working Directory Name is "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\ExtensibilityData".  
> GetSymbolicLinkPath: Adding ProgramData to symbolic link path  
> Deleting existing symlink:'X:\ProgramData\MSSQLSERVER\Temp-R'.    
> User account pool creation encountered failure. Removing user account pool changes.  
> Deleting user account configuration.  
> Working directory from data root "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL" is "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\ExtensibilityData".  
> Working Directory Name is "X:\Program Files\Microsoft SQL Server\MSSQL14.MSSQLSERVER\MSSQL\ExtensibilityData".  
> GetSymbolicLinkPath: Adding ProgramData to symbolic link path  
> Deleting existing symlink:'X:\ProgramData\MSSQLSERVER\Temp-R'.  
>
> Unable to get working directory root for cleanup.
>
> Error: Couldnt create the symbolic link for ExtensibilityData  
> 
> Removing firewall rule for user account pool.  
> Removing user accounts.  
> Processing 020/020  
> Dropped SQL login for the R SQL user group [my user group]  
> 
> Error: Couldnt create the symbolic link for ExtensibilityData  
> Failed to complete the operation successfully.

How do I fix this?