When I try to test purchase an In-Skill Purchase for my Alexa skill in beta, I get "Sorry, I am unable to fulfill your request on this device" as a response on my iOS device, but on my Echo Dot is works perfectly fine. I can't seem to find any documentation regarding ISP unavailability on mobile devices. Any idea as to why this is happening? Thanks in advance!
According to this exchange from the forums, an Alexa Developer Advocate confirmed in November 2019 that ISP purchases could not be completed from mobile devices.