This FAQ applies to Avid Studio and Studio 16.
If you have a PAL timeline with Mercalli applied to it export to an NTSC format, the Mercalli effect is not properly re-analyzed, causing the effect to not be usable. As a workaround try changing the project timeline to NTSC and save the project under a new name. You can change the format by clicking the Timeline Settings Gear button in the upper-left of the timeline. Once that is done, export the project.