how do I capture a single frame from a video in Photos?

I have Photos 7 and Monterey, and followed the instructions. But in Export, the Export Frame to Pictures is dimmed.

Which version of Photos are you using? In Photos 7 do it like this:

How to extract a still frame from a video… - Apple Community

The method has been described in the user guide in the section Export photos, videos, and slideshows from Photos on Mac - Apple Support of the Photos user guide, the paragraph "Export a video frame as a photo":

You can review a video clip to identify an outstanding frame and export it as a photo into the Pictures folder.

1. In the Photos app  on your Mac, open a video clip, and drag the playhead to the frame you want to export.
2. Choose File > Export > Export Frame to Pictures.
3. A TIFF file named “Frame [date]” appears in the Pictures folder.