If it is the Paragraph Style dialogue then I can’t enter anything in there on Windows 11, LO 188.8.131.52, 184.108.40.206, & 220.127.116.11
If you have some text in a document and select a couple of paragraphs, then click Format > Paragraph (that is NOT style dialogue) and enter a word, for example Zebra, then you will see what Zebra would look like in drop caps. If you then OK the dialogue then the first 5 letters (number of letters in Zebra) of the paragraphs become drop caps and Zebra is not shown.
Oddly, this is not what Help implies:
Enter the text that you want to display as drop caps instead of the first letters of the paragraph. but it seems to work as I expect.