The Galaxy A15 does not have a built-in app lock feature in its default settings. However, you can use third-party apps from the Google Play Store to add this functionality. Here’s how you can do it:

1. **Download a Third-Party App Lock**:
- Go to the Google Play Store.
- Search for "App Lock."
- Choose a highly-rated app such as "AppLock" by DoMobile Lab or "Norton App Lock."

2. **Install and Set Up**:
- Install the app you selected.
- Open the app and follow the setup instructions, which typically include setting a password or pattern.

3. **Lock Your Apps**:
- Select the apps you want to lock from the list provided by the app.
- Configure any additional settings, such as fingerprint unlock or delay relocking.

These third-party apps will allow you to lock individual apps on your Galaxy A15, providing an extra layer of security.