MacBook Pro (13-inch Early 2011), macOS Sierra (10.12.6)

Which version of iMovie do you currently have installed?

Are you seeing this message?

This is just a warning, that the app may no longer be compatible in a future system version, that requires 64-bit applications, see:

32-bit app compatibility with macOS High Sierra 10.13.4 - Apple Support

You might be seeing this warning for iMovie 9.0.9, but not for the current version iMOvie 10.1.8.

Getting error message that iMovie is now "incompatible

But what is the exact error message you are seeing - literally?

The most recent version of iMovie 10.1.9 requires macOS 10.13.2 or newer, but if I remember correctly, your version should be able to run on your system version.

What happened, before you saw the error message? Did you downgrade the system from High Sierra to Sierra?

I have seen the update at the AppStore just two hours ago. You may want to check the updates tab at the AppStore.