The server could not be reached. Please contact your system admin.
This issue can appear if you are trying to login before all the services have fully started after a reboot.
If the issue persists after 15 minutes from startup then review the following NetWitness Knowledgebase article.
Reference: Error message "The server could not be reached" is displayed when attempting to log in after upgrading to RSA NetWitness Logs & Packets 11.0
The article suggests checking the rsa-nw-admin-server service is running on the NW Admin Server appliance and restart it.
If this doesn't lead to a resolution of the issue then call your local NetWitness Support number to create a case to investigate the issue further.
Please be aware that NW 11.5 will reach end of product support after September 2022.
Reference: Product Version Life Cycle for RSA NetWitness Platform