WELL because Reasons they ended up fighting, full on battle (+ lesser variants, depending on the situation), like, a lot. Error is a big ball of spite. Sometimes tho, they hung out because lonely, but mainly Error wanted to kill him. It's complicated. Ink wanted to get to know him and maybe not fight.

Then Shenanigans went down and Ink sort of ruined his own life

And then he hunted down Error who was like "wow dude you screwed up also im going to kill you"

And ink was like "yeah i did and probably"

Then they dodged and fought a lil then just sort of. stopped. and sort of developed a neutral, unspoken, "lets just stop and chill" truce

And theyve been stopping and chilling ever since

Also the everything is pretty much dying but thats another story

Actually no its not but you get the idea