On Android version 6.0 or later, "Draw over other apps" permission is required to display the Truecaller caller ID. You can give Truecaller this permission by following these steps.
Enable Caller ID and spam identification from Truecaller Settings
1. On your device, open Truecaller app
2. Tap on your profile icon > Settings > Caller ID
3. Click on Allow Truecaller to appear on top of other apps and you will be redirected to Display over other apps system setting
4. Look for Truecaller and select the option
5. Toggle the button Allow to display over other apps
Enable Caller ID and spam identification from device settings
Alternatively, you can follow the steps mentioned on this page to give Truecaller this permission via your device settings.