How to remove flickering caused by LED screen

You will have flickering if your camera's frame rate does not match the screen's refresh rate. There is no sync available with a GoPro.

The only hope you have is to match the frame rate to half the frequency of the electricity (50 Hz in Pal countries, 60 Hz in NTST-most of the world) and have anti-flicker in the camera set to the proper Hz. You can mitigate the problem by choosing slower shutter speeds. 

A lot of those kinds of LED displays have very odd refresh rates. Reader boards at airports and train stations can be anything from 10 or 15 Hz to 50 or 60.  I don't know of any de-flicker software available that would fix your shot. The flicker is not uniform, and I didn't see any frames with a full image. It looks like about 1/3 of the display is lit up at a time. That's not uncommon in that kind of display.

Your problem fascinated me, and I was able to solve it fairly well by following this workflow. 

1. Create a comp from the original footage and add Mocha AE
2. Open Mocha AE and create a spline that covers the left corner of the display and the right corner (Ctrl/Cmnd + g to add to the spline)
   
3. Track Translation, Scale, and Rotation and then select the surface tool and expand it on the first frame to fill the frame, save and return to AE
4. Duplicate the footage layer, add a "Stabilized Null to the timeline, select the Copy of the Problem Shot with mocha applied and tne null, and Pre-compose moving all attributes to the new comp
5. Collect the tracking data from the Stabilized Track, invert it, and apply it to the Stabilized Null
   
6. You should have a Stabilized shot of the footage where the sign does not move
7. Return to the main comp, Pre-compose moving all attributes, name the comp "Blended Sign," and open it
8. Draw a mask on the layer to include only the sign, then duplicate the layer twice
9. Set the bend mode of the top two layers to screen, go to the first frame, then slide the second layer one frame to the left and the top frame one frame to the right so they are slightly out of time with each other
   
10. You should have a sign that looked pretty good. Return to the Main comp and enable time remapping, then stretch the time a few frames to help blend frames. Setting frame blending to Pixel Motion seems to work best
11. Turn off the Invert checkbox in Mocha AE/Tracking Data and apply the tracking data to the Blended Sign comp

That's it. You should get something like this:

The last step is to add Effects/Color Correction/Shadow Highlight to the Blended Sign layer to bring back the natural lighting highlights in the sign.

I hope this helps. It took me just a little longer to do than it did to write this post. I've uploaded an AE 2024 project file and the rendered fixed movie.