I'm installing a new faucet and it's going fairly smoothly except I cannot seem to get this adapter (unsure that's the right word) to stop leaking.

I've managed to tighten the supply line (top green circle) and bottom nut (bottom green circle) enough where there are no leaks there, but this nut in the middle (red circle) will not stop leaking from underneath (see blue lines; I'm an MS Paint master). I've taken the supply line off and tried tightening just the nut and it will get gradually tighter until it's loose and I can finger tighten it again

This adapter was installed with the old faucet which I tried to repair, failed miserably, and have since removed. Do I need to replace this part or is there a way to tighten it I am just not getting?

I'm installing a new faucet and it's going fairly smoothly except I cannot seem to get this adapter (unsure that's the right word) to stop leaking.

I've managed to tighten the supply line (top green circle) and bottom nut (bottom green circle) enough where there are no leaks there, but this nut in the middle (red circle) will not stop leaking from underneath (see blue lines; I'm an MS Paint master). 

I've taken the supply line off and tried tightening just the nut and it will get gradually tighter until it's loose and I can finger tighten it again

This adapter was installed with the old faucet which I tried to repair, failed miserably, and have since removed. 

Do I need to replace this part or is there a way to tighten it I am just not getting?

I'm installing a new faucet and it's going fairly smoothly except I cannot seem to get this adapter (unsure that's the right word) to stop leaking.

I've managed to tighten the supply line (top green circle) and bottom nut (bottom green circle) enough where there are no leaks there, but this nut in the middle (red circle) will not stop leaking from underneath (see blue lines; I'm an MS Paint master).

This adapter was installed with the old faucet which I tried to repair, failed miserably, and have since removed. Do I need to replace this part or is there a way to tighten it I am just not getting?

I'm installing a new faucet and it's going fairly smoothly except I cannot seem to get this adapter (unsure that's the right word) to stop leaking.

I've managed to tighten the supply line (top green circle) and bottom nut (bottom green circle) enough where there are no leaks there, but this nut in the middle (red circle) will not stop leaking from underneath (see blue lines; I'm an MS Paint master). I've taken the supply line off and tried tightening just the nut and it will get gradually tighter until it's loose and I can finger tighten it again

This adapter was installed with the old faucet which I tried to repair, failed miserably, and have since removed. Do I need to replace this part or is there a way to tighten it I am just not getting?