As you are using self signed certificate so in that case it is normal for browsers to show the warning message.
You can proceed by clicking on "Advanced" --> "Proceed to 192.168.6.157 (unsafe)".
Then you should be able to access NiFi UI
If it is still giving the error then your Certificates may be not setup correctly. Hernce it might be causing:
192.168.6.157 didn’t accept your login certificate, or one may not have been provided.
Try contacting the system admin.
Have you stored any certificate in your browser? May be there is some issue with your certificate. May be you can provide the exact steps which you used to implement NiFi SSL will give us better idea on what is going wrong.