Is it new Airwave installation server or old, are you not able to delete any down device from Airwave server aswell?
May I know the version of Airwave, if your server is in below 8.2.4 version code then you can try recompile code by running make command but before running make , it is better to check
/var/log/httpd/error_log and /var/log/nginx/nginx_error.log logs under system>status page for any error entry while trying to delete or add device.
If your server is newly installed Airwave latest version then first check logs to see for any error and later we can try recompile code but for that you need to take TAC help. We can not run make command from AMPCLI.
Try restart AMP services once from system status page aswell.