The <Take> wildcard in the file name will increment once for each recording session, helping you distinguish attempt #1 from attempt #50 at recording the same sequence.
If you've selected a video format as your output, that's enough. All the frames for a given take get packed into the same single video file.
But when you switch to outputting a still image, just a <Take> number isn't enough: now we need a separate file name for each frame, so saving the next frame doesn't overwrite the previous.
You can click the "+ Wildcards" drop-down to add a <Frame> wildcard to your filename, or type it out yourself. This will increment for every frame in a take, keeping the latest frame from overwriting the previous, and ensuring that you can later reassemble the frames in the right order.
Unity doesn't add this automatically because it doesn't know where in the file name you want it. Many game studios use pipeline tools that expect a very specific file naming convention to work, so having Unity automatically alter the text in that box when you change another setting somewhere else in the dialog would be an easy way to get sneaky bugs that silently break your pipeline.
