Need to configure TACACS+ on LTM BIG-IP 11.4.1

I enabled tacacs+ for BigIP LTM 11.6.0 3.0.412 last week against ACS5.5. It worked right away with below configuration.

Under System ›› Users : Authentication I put following settings:

• User Directory Remote - TACACS+
• Encryption Enabled
• Service Name ppp
• Protocol Name ip
• Authentication Authenticate to first server
• Accounting Information Send to first available server

And of course the tacacs+ servers with their shared secret.

Are you sure you are running ACS2.6? Because this is quite an old release and end of support for more than 10 years...