leaking around Lock nut where water goes into toilet tank

[satire70]

Hi
I need help.
there are drops of water around the black Lock nut.
First I thought it was leaking from the toilet tank, so I have tested having
full water in the toilet tank and watched overnight. and the water hose was not conneted to the Lock nut.
Then there are no water around the Lock nut.

It only happens when the water hose was connected. If I did not connect the water hose right, I understand if the water is around the bolt(screw?), but it is above the bolt, the Lock nut.
How the water can get up thinking the toilet tank is not leaking?
Can anybody help me?

from a desperate mom

 

[Terry]

If you are using a flexible connector for the toilet tank, make sure it's clean between the rubber seal and inlet of the tank fill valve.

If you have any TEFLON® tape, remove it now. The TEFLON® will not help in any way, but is very good at causing leaks.

Sometimes I think my job as a plumber is to remove TEFLON® tape everytime I see it.