Got the same error on wildfly version 16
Error
Failed to connect to the controller: The controller is not available at localhost:: java.net.ConnectException: WFLYPRT0053: Could not connect to remote+http://localhost:. The connection failed: WFLYPRT0053: Could not connect to remote+http://localhost:. The connection failed: Connection refused
And following done and resolved successfully
Step 01
Comment (or you can remove) following line from /bin/jboss-cli.xml
default-protocol use-legacy-override="true">remote+https</default-protocol
Correct protocol Ex:
<default-protocol use-legacy-override="true">remote+http</default-protocol>
<!-- The default controller to connect to when 'connect' command is executed w/o arguments -->
<default-controller>
<protocol>remote+http</protocol>
<host>localhost</host>
<port>9990</port>
</default-controller>
Step 02
In my case i have alredy created a Administrative user hence, I have statup the CLI with following commnad
./jboss-cli.sh --user="<user>" --password="<password>" --controller=remote+http:<your IP>:<port> --connect
Example :
./jboss-cli.sh --user="Admin" --password="Password" --controller=remote+http://19.199.115.172:9990 --connect
Make sure your wildfly is up and running. If you have used different port for the admin console it should be added .