Heyy. Some time ago I showed how there were lines that appeared at the node level when using linear gradient. I tried to make the same vision using mesh gradient as well but sadly there appears the same problem :( Does anyone know how to fix this? 

Maybe 25-50% Blur then clip so the blur doesn't bleed into surrounding areas. You will also have to duplicate the circle with no fill and place it on top. 

Maybe?

Between the steps the interpolation is linear. You need to add more steps for a smoother look.

If your concern is about the Mach banding, you can add some dithering. 

Use a filter with displacement map, something like these: https://openclipart.org/detail/272359/dither-filter-pack

The circled line is the one I want to get rid of. Does this happen to everyone?

Yes, it happens. The gradient uses linear interpolation.

In gimp or blender, you can set the type of interpolation between the steps.

In illustrator, you can adjust  the influence of a gradient step for better ease.

Attaching an example svg with a second gradient, witch has additional steps. 

Maybe not perfect, but can show the idea.

I tried using the filter you sent me and it still didn't work, even at 10, 20 or 30 opacity :( I tried multiple things like downloading Gimp, where the same thing happens, using noise textures, watching youtube videos, searching on google and nothing. I'm just confused on how professional graphic designers make this nonexistent, if they even do.