iOS 11.3 not working

Hi.

If an error occurs during upgrade using wireless connection, then you may try using iTunes on a computer having Windows or MacOS.

I think you may not have enough space on your iPhone to install the upgrade iOS 11.3.

If you install the upgrade on a computer, the installation goes faster because you use your computer's resources, CPU, RAM, space, etc. and a modern computer is faster.

I installled this upgrade last night without any error on my iPhone SE using iTunes on my Windows 10 PC.

Hope this helps.

Try updating using iTunes as described here.

Update your iPhone, iPad, or iPod touch - Apple Support

To ade257