Can I pair my IPhone 6plus with Apple Watch 5 by any way regarding last IOS version is 12.4.3

You cannot use an iPhone 6 Plus with an Apple Watch Series 5.

You should return the Apple Watch Series 5 within the Apple 14 day return period for a refund.

In the future when you have a newer iPhone you can buy an Apple Watch.

The Apple Watch requires an iPhone 6s, SE, 7, 8, X, XS, XR, 11 or 11 Pro

The current Apple Watches being sold come with watchOS 6, which requires iOS 13, and iOS 13 only runs on the above list of iPhones. The iPhone 6 Plus cannot be upgraded past iOS 12

I'm afraid not the apple watch 5 comes with watchos6 and this needs a phone that has ios 13 or higher.