This seems to be a issue that a different version of MS SQL plugin was deployed on the Agent side.
As per your description you have already downloaded and deployed the MS SQL Server Plug-in. I am assuming that the version of plugin is 18.104.22.168.
If not you will need to deploy 22.214.171.124 MS SQL plugin on OMS first and then on the Agent which is monitoring the MS SQL targets.
+ Command to get the plugins deployed On OMS server currently,
$ cd <OMS_HOME>/bin/
$ ./emcli login -username=sysman
$ ./emcli sync
+ Command to get the plugins deployed On Agent currently,
$ cd <AGENT_HOME>/agent_inst/bin/
$ ./emctl listplugins agent
Ref: EM 12c: How to Verify the Plugins Deployed on the Enterprise Manager Cloud Control 126.96.36.199 OMS and Agent (Doc ID 1431103.1)
You can review the below link for MS SQL plugin deploy on OMS and Agent,
Once the 188.8.131.52.0 MS SQL plugin is deployed on OMS and on Agent try to dicover the targets again from console. I hope it will help.