My phone will not back up

If you are getting a message that there isn't enough power, is the phone connected and charging while you are backing up? Is the 2 bars your cellular signal or your WiFi signal? Backups are done using WiFi.

See if anything here helps

Backup couldn’t be completed.   *

Backup couldn’t be completed or you can’t restore.  *

Backup couldn’t be completed or you can’t restore (2).  *

Backup couldn’t be completed or you can’t restore (3).  *

If you can't back up to iCloud - Apple Support

Backup methods for iPhone, iPad, and iPod touch - Apple Support

It would help if you included how you are trying to backup, what equipment is involved, and any error messages you see.