Popup User Notification

For notifications I have a policy that executes a script that uses jamfhelper and scope it to the devices. Scopeing to users will probably not work in our case since most users use local accounts but for Self-Service we use AD accounts.

@mickl089 Do you also have a Startup Script configured? Do other policies with a Startup Trigger work as expected?

@mickl089 I don't think you should be using the Startup trigger for a policy that would display a message to the user. Startup literally means when the Mac starts up, which is not the same thing as login.
If you explain what the end goal and purpose is for a message to display this early on at each boot (you have your policy set to Ongoing), then maybe there's a better way to do this, such as a Policy Banner?

@mm2270 I want to set that a user gets a window or a message after login that he should update to macOS Big Sur. Just a little hint that you can also end with "OK" :-)

Ok, so in that case, you could try enabling Login / Login hooks in Jamf and set the policy to Login instead of Startup. As I mentioned, typically Startup isn't a good trigger to use for a user type message because it's likely the policy is firing off before an actual login to the desktop has happened. Be aware though that a lot of folks have problems with login policies these days, in that they often don't work properly, so YMMV on using that trigger.
If you find it's also not working for you, you could consider using a LaunchAgent and script deployed to the Macs that you want to target for this. You'd have to manage this in a way that after they upgrade to macOS Big Sur that the LaunchAgent and/or script gets removed or else they'll get the message coming up again. LaunchAgents run as the logged in user, meaning they don't actually activate until a login happens.

You can call up messages using things like Applescript (osascript in a shell script) or jamfHelper (/Library/Application Support/JAMF/bin/jamfHelper.app) or even just using the jamf binary's displayMessage verb, just to name some examples.

If you need help crafting some of the above, post back and I'm sure one of us can put something here to get you started.

If you can scope to a smart group, that could trigger a policy with a Files & Process payload that would execute a command like this - /usr/local/jamf/bin/jamf displayMessage -message "Inset your words here!"

For example, we have a smart group for devices with uptime >14 days. We trigger a message when they enter the smart group and weekly so long as they remain in the smart group.

If you wanted something more aligned with macOS notifications, then perhaps something like this - DataJar Notifier