When I attempt to do so, a prompt appears telling me to update my phone's iOS. I am wondering whether there is a workaround; I do not want to lose my jailbreak. Thank you in advance.
WatchOS 2 is supported only on devices running iOS 9 and higher.
The only way to run an Apple Watch using an iPhone running iOS 8.4 is to have an Apple Watch with watchOS 1.x.