Single User Unable to Connect to VPN over F5 APM

Hello HarshaNK​ 

Access-Reject should come from Radius
You see loopback addrees cause i guess you have a Radius Server Pool and not a direct Radius Server.

You should therefore see logs on Radius side.
You could also capture traffic to be sure that request reached Radius Server

Moreover, as it happens for a specific user, check if there are non-ASCII characters on username or password