Wall Error issues that seem to glitch normally? [closed]

I'm actually very curious on simple things that I seem to STILL not understand and I've looked everywhere and now I'm actually pretty lost to be honest.

1: Whenever I make a hole in a wall I notice it refuses to let me put Decals on it which is messing with what I'm making greatly.

2: Whenever I try to connect a wall together I notice it never looks "High Quality" level, it seems to have a error and makes it where it looks like it's blinking really fast. Although it's just one wall glitching with the next. (HARD to explain).

I notice in Jail Break & other games the walls look crispy clean with no errors. So I'm clueless. This will be greatly helpful to know as it is my main issues.