I encountered problems with spellcheck in InDesign 2020. Apparently this issue has been reported over the last 3 years or so.
If spellcheck does not work, go to Preferences > Dictionary. What you will probably see for HYPHENATION an SPELLING is PROXIMITY. Change proximity to HUNSPELL, and spellcheck should work (this worked for me). If you want to use AUTOCORRECT, go to Prefs > AutoCorrect and make sure ENGLISH US is selected for the dictionary; mine was set to German.
Maybe this will save someone a bit of frustration.
Instructions to subscribe or leave the list are at https://www.gnsi.org/listserv
List name is Sciart-L
Problems: Email Lana Johnson at [log in to unmask]