How to create a video that can be uploaded to Twitter in Mac?
From Twitter's support page:

We currently support MP4 and MOV video formats on mobile apps.
On the web, we support the MP4 video format with H264 format with AAC
audio. You can upload videos up to 512MB, however you will be prompted
to edit videos to 2 minutes and 20 seconds or less in length.

You can export an h264 format video directly from Final Cut.