Why won't by ios11 download ?

Hard to tell, you do not provide any information. How were you trying to download iOS 11? What iOS devices are you using? If you are trying to update the device, see this, Update your iPhone, iPad, or iPod touch - Apple Support

If you are having troubles with over the air, see this, Get help with over-the-air iOS updates - Apple Support

I have the same issue. Plenty of space, battery 98% and wifi connection but will not download after I enter the login code. Tried restarting and closing but no help.

Cant connect over itunes tunes cable as home PC crashed

In my first reply to the OP, there is a link for problems with OTA updates. If that does not correct the issue, then you are going to have to borrow a computer, or go to the nearest Apple store and get some help from a computer of theirs.

Have you tried to update using iTunes?