Unable to change clip speed

I'm having some weird behavior attempting to change clip speed. I'm new to DaVinci Resolve, but I've performed this step before with no issues. Now I can't figure out why it's not working.

I have a simple project with one clip in the timeline. It's an MP4 downloaded from YouTube. Also tried with an iPhone video. I'm right-clicking a clip and choosing "change clip speed." When I try to type in a new duration, I can't edit the digits.

Thanks in advance for any help!

[Peter Chamberlain]

It could be a variable frame rate clip or some other technical issue.

If you like, post a link to the clip so others can check.

Also, try converting/rendering it to more pro format (DNxHR) and then replace the current timeline clip before speed changing.