Are you getting that error message while configuring LDAP for your splunk environment. And you have already tested that your generic user credential are working fine by logging as domain user.
if this is the case then:
Make sure you have all the correct details required for this configuration. You can get this by contacting your AD administrator.
Host: AD server IP/Hostname OR Load balancer URL.
Bind DN: distinguishedName of that Generic ID/user. in your case i think your user is "splunktest".
Bind DN Password:
User Base DN: This is nothing but distinguishedName of container under which all domain users are listed.
Group Base DN: This is nothing but distinguishedName of container under which all your groups are listed.
Share your configuration details if you don't mind.