Unusual excuse / reason
Such tickets can be purchased at any manned station
More likely the staff member didn't want to waste time trying to find it!
Equally, not on commission?
What is this obsession with staff being on/not being on commission? That has absolutely nothing to do with the ability/willingness to issue a ticket in this scenario.
Anyways, rovers are available from any ticket office. Unfortunately, some staff don't know how to issue them. Having worked in a ticket office in London, I was never once asked for a rover or ranger. I did know how to issue them, anyone with two brain cells to rub together and a customer focused attitude should be able to locate the menu within the TiS they use. Having said that, if I was told I could not do something, I'd check with a manager first. Sometimes, you just don't know if there is some unknown rule in place that can get you told off. I mean, I could go to Forest Hill ticket office and buy one of those Northern Only £10 rovers. The clerk may well get into trouble for it as it would be in contravention of the TSA, but there's about a 0.0001% chance they would have been told not to sell it.
I wouldn't attribute it to poor training - with tickets, it's impossible to be trained on everything and even if finer details were included in training, the majority of people would forget about the minor things that they never have to use. Not everyone has an enthusiastic interest in the minute intricacies of the ticketing system. So no, it's not so much bad training as having a generally poor attitude. Like I said, working a ticket machine isn't rocket science, sometimes it just requires a bit of patience. There were things I didn't 'know' how to do the first time (things not offered on that line of route, such as cycle reservations and specific seat reservations for example), so I'd ask the customer to wait whilst I found out how to do it. Normally, people asking for unusual things are understanding of this, though it wouldn't surprise me if some of the people on here would aggravate the situation by having a bad attitude towards staff who don't know everything off by heart.