unable to update ios 26.6.1

I have an iphone 17 pro with ios 26.6 to update to ios 26.7.1. unfortunately I go no further than 'update requested' or 'requesting' ; it last a long long while until a window appears: the update couldn't be installed'...

please help, what can I do ?

i must add that I have a vpn that is switched off completely...

chris