Why I cannot update my iPhone to the latest version 15 ? I use iPhone 8 plush

Can you post a screenshot, it could reveal why you aren't able to update.

Make sure you:

• Are using a WiFi connection (not a hotspot).

• Have your battery charged at least 50%. Better yet, keep it plugged in during the update.

• Are not using a VPN connection.

• Do not have power save mode turned on.

• Do not have a beta version installed.

• You have at least 6 gigs of storage available.

You can also check out this article for help -

If your iPhone, iPad or iPod touch won’t update

https://support.apple.com/en-gb/HT201435

You need both cellular service and WiFi to do the updates.