You can lock any apps you want to safeguard your privacy.
Here's how to lock apps on OPPO A37 (A37fw):
Step 1: Access the 'Security Center'.

Navigate to the 'Security Hub'.
Step 2: Select 'Privacy Settings'.

Opt for 'Privacy Settings'.
Step 3: Choose 'App Encryption'.

Select 'App Encryption'.
Step 4: Choose 'Enable Encryption'.

Opt for 'Enable Encryption'.
Step 5: Set up the lock password.

Set up the lock password.
Step 6: Select security questions.

Choose security questions.
Step 7: Select the app you want to lock.

Select the app you wish to lock.
That concludes the setup process. Wishing you all success.