Finished downloading, Why cannot proceed installing IOS 15

To update all of these requirements must be met:

• You must be connected to a Wi-Fi network (and not a personal hotspot) unless it is an iPhone 12 series with an active 5G network connection
• The phone must be charged to at least 50%, and preferably plugged in so it stays above 50% during the update
• The phone must not be in Low Power Mode
• The phone must not be connected through VPN
• The phone cannot be in Airplane mode

Can some one help me plz

Are you on charge and greater than 50% charged ?

I Have downloaded Ios 15 but getting isuue while installingg ...

See the reply from Lawrence Finch

If you post the entire screen shot and not cut off the top and bottom it would be much easier to help you diagnose the issue.