I have a screenshot. I want to highlight some parts of it. The highlight icon is disabled; the
Tools > Annotate > Highlight Text is greyed out.
Why? And how can I highlight easily on my screenshot
You can't highlight text because the format of your screen capture is a bitmap (a set of pixels and not text).
Nonetheless, if the text you wanted to highlight is horizontal enough and not too distorted, you can achieve a nice highlighting effect with
Tools > Annotate > Rectangle. Then choose a color which isn't frequent inside your image, let's say
Lemon and decrease the opacity to less than 25 % to be able to see the text through as in this example:
If your need is important in volume of text to highlight, you will have to use an OCR software to transfom your image into text.