“The ACS appliance is not available for one of the following reasons: either it is not running, it is in use by another DSView system, it does not have the right firmware version to communicate with the DSView 3 or ADSAP2 is not in trust-all mode.”
What can I do?
There are different things that may cause this problem:

ACS is not on trust-all mode. It happens when it was added to another DSView before.

The plug-in and the ACS firmware are not compatible

DSView server can’t contact the ACS through the network. Firewall issue

Things you can do to fix it:

Connect to the ACS via telnet or ACS and enter the command

# adsap2_clear
It will let the ACS be added to a new DSView server.

Login into the DSView server (via SSH or RDP) and try to open a SSH connection to the ACS from there. It will show that the IP communication is fine.
Also check if all required ports are opened on the firewall. They are:

TCP: 443, 1078, 22, 3211, 2068, 8192 and 3871

UDP: 3211

Read the ACS plug-in release notes and see if the firmware is compatible. Avocent always recommend running latest plug-in and firmware.