how can I install ios 12 on my device, when all i get offered is ios 15.6?

how can I install ios 12 on my device, when all i get offered is ios 15.6

Only the latest version of iOS is offered for download and installing which, currently, is iOS 15.6 for iPhone.

how can I connect apple watch 3 with iphone se who has ios 10

As Apple advises, update your watch and your iPhone to the latest software (iOS 15 and Watch OS 8), then follow the instructions in Set up your Apple Watch.

Your iPhone SE can install iOS 15 - see iOS 15 and iPadOS 15 are compatible with these devices - and the Apple Watch Series 3 can install Watch OS 8 - see watchOS 8.

What, specifically, is "something". What is the error message telling you? Do you not have enough space on your iPhone, for example? Are you using Wi-Fi or attempting an update via your phone data? The latter will not work on your device, which I presume is now a good age.