The "er" is a hesitation, and indicates that Tatclass realises he's made a mistake in relaying the previous line to the community. "hi" is a sheepish greeting (in this case).
You've probably seen television programmes where the presenters are joking amongst themselves and only belatedly realise the cameras are on them.
Andy smooths over the insult by allowing that it's a typo, an easy mistake to make.
Tatclass acknowledges that by saying that the keys he used to type the line in capitals are all next to each other.
The joke is that they are not: Andy's response is actually a form of irony: he's feigning ignorance. Tatclass probably typed his initial line into the wrong window.
Irony (from Ancient Greek εἰρωνεία (eirōneía), meaning "dissimulation, feigned ignorance"), in its broadest sense, is a rhetorical device, literary technique, or event in which what appears, on the surface, to be the case, differs radically from what is actually the case.