Yes. I couldn’t do it on the website a year or 2 ago due to having selected an exit row seat; the website did not give the reason, just wouldn’t do it. Called, phone rep told me the reason, changed my seat and booked the pet fee. It may be that pets are not allowed for other seats (? bulkhead) and the number of pets per flight and per cabin (First, Economy) is limited. Perhaps the website simply will not allow a pet fee to be booked when such a seat has previously been selected or cabin pets maxed out, without giving the reason. More recently, after selecting non-exit row and non-bulkhead seats, I’ve not had any problems booking pet fee on-line.