That is not a valid formula… no values are accepted inside the parentheses for NOW() and TODAY().
An easier formula would be to check if the current hour is less than 19:
HOUR(NOW()) < 19
Note that this will evaluate to GMT time, so you may need to change the number away from 19 to get the hour (in GMT) that you’re looking for.
Also note that
NOW() doesn’t update in real time — it updates every 15 minutes or so when the Airtable base is open, and every 60 minutes or so when the Airtable base is closed.