I see this error as well - that is why I'm on this thread, reading through it ....
I'm trying to reconcile this thread with Coinbase's recent email. Are we talking about here the User Activated Soft Fork (UASF) that Coinbase is referring to?
Coinbase also refers to User Activated Hard Fork (UAHF) that is scheduled for Aug, 1st. They are not supporting this fork.
This error is related to the UASF - which does not require a software upgrade on wallets of core 14+ (or some 13s versions), and Coinbase is supporting?
This thread is talking about neither of those things. BIP 91 is unrelated to UASF or UAHF. Neither the UASF or the UAHF will trigger the warning that you see here as they do not do miner signaling; rather they activate via flag day. However BIP 91 activates via miner signaling, so we get the warning here about that.