How do you lock your phone. I can’t back up

Locking the phone is simply pressing the sleep/wake button. In other words, when your phone goes to sleep, it is locked.

If you want to automatically backup with iCloud every day, your phone has to be locked, connected to WiFi and plugged into charge.

Have you tried a manual backup? Settings>Your Name>iCloud>iCloud Backup >On>Back up now.