When I insert this below link like GeoServer 2.17.1 RESTful API I got only: userName%
curl -u admin:@dmin9901 -X POST http://localhost:8080/geoserver/rest/security/usergroup/users/ -H "accept: application/json" -H "content-type: application/json" -d "{ \"userName\": \"test\", \"password\": \"test1234\", \"enabled\": true}"
It can't create new user in my GeoServer, it would be bug?
Looking at the API docs it looks like the API endpoint you are hitting is incorrect.
Based on this I would expect the call to be:
curl -u admin:@dmin9901 -X POST http://localhost:8080/geoserver/rest/usergroup/users/ -H "accept: application/json" -H "content-type: application/json" -d "{ \"userName\": \"test\", \"password\": \"test1234\", \"enabled\": true}"
If that is not the issue I'd look in the geoserver log file (by default it will be in $GEOSERVER_DATA_DIR/logs. As well as adding the -v option to the curl command to see what the response headers look like.
