[grace-baker Hexnode 2 December 2019 grace-baker Moderator]

Hey, Thanks for reaching out. We are here to help you.

This may be because the policy associated with the device is restricting the app installation from unknown sources.
To ensure that the option Install apps from unknown sources are enabled, please navigate to Policies–>Android Settings –>Advanced Restrictions–>Allow App Settings. Enable the ‘Install apps from unknown sources’ option

PS: This is supported on Samsung Knox, Android Enterprise-Profile Owner, and Device Owner mode

Kindly use the link for reference.

Cheers!
Grace Baker
Hexnode MDM