Hi,
This is probably one of two things:
a) XPages is not enabled to run in your Domino server config
Run Administrator and for your server do:
Configuration > Server > Current Server Document > Security > Programmability Restrictions > Enable XPages blah
b) Your application contains code that is not signed by an approved user id
From Administrator, you could go to Files and locate your NSF and sign with the appropriate ID using right click menu
Designer offers this feature also since 8.5.2 - Right click NSF and it's something like Application > Sign Design
The latter will use the currently logged in ID
After doing these, restart the server and try to run your app again
-M