Why curl returns "userName%" by GeoServer REST API create new user

Looking at the API docs it looks like the API endpoint you are hitting is incorrect.

GeoServer API

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.