Phone screen sometimes doesn't respond to touch

You'll want to try any and all software solutions first, up to and including a full factory reset and/or restore of the Android operating system on your phone, but if that still doesn't fix the issue then the most likely cause would be a failed or damaged digitizer. Has there been any damage to the phone recently, such as being dropped or getting wet?

The digitizer and screen are fused as one part so you have to replace the entire assembly in order to fix a bad touch screen. Unfortunately, they don't seem to have a guide for your phone here on iFixit, but there's a French site called SOSav that does.

Full screen (screen + frame) Galaxy a30 repair - Free guide - SOSav

You can find replacement screens with or without the frame on such sites as Amazon, eBay and AliExpress, among others. The guide I listed above only shows you how to replace the screen + frame assembly, but you can also buy just the screen without the frame; it's a little cheaper (but not much) and you don't have to transfer all the parts from one frame to the other, but I didn't run across a guide for just the screen replacement without the frame. An internet search will probably turn up something if you decide to go that route.