You can enable the obscure origin feature to install applications from external sources.
To activate mysterious origins on Asus Zenfone 3 ZE520KL, follow these steps:
Step 1: Go to 'Settings'.

Access 'Settings'
Step 2: Choose 'Security'.

Select 'Security'
Step 3: Enable 'Unknown sources'.

Enable 'Unknown sources'
Step 4: Choose 'OK'.

Select 'OK'

Wishing you all success.
