New apps fail to install

ASUS Max Pro M1 Android 9 Not rooted
Recently the phone was updated to 9 and after wards I am unable to install several applications. The installer just exits informing not installed. I also checked settings and find that the setting for permitting installation of applications from other sources is missing.
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Hi there, kindly share latest firmware version of your device.
also, you may follow the below steps:
1. Clear system cache.
2. Try the same under safe mode:
Hold the power button for 5 seconds => When the screen flashes with option => Press & hold power option => You will get an option to restart in safe mode
3. If still you face the issue
factory reset your phone after taking a full backup : https://www.asus.com/support/FAQ/1006346/
4. Last option would be to visit the service center to have
a check. Locate here: https://www.asus.com/in/support/Service-Center/India