I agree, it's not calculating.
I have entered my amounts under employment expenses for each relevant line on the T777S but nothing is flowing to line 22900. Maybe its not working because we are also supposed to also enter the square footage of the designated room over the total square footage to determine the percentage to apply. However I don't see where this would be entered.
It seems that turbotax still needs to update this and hope they see this and fix it soon!
I figured it out. At the bottom of your T4 entry, you need to enter yes for the following question:
Will you claim employment expenses for working at home due to COVID-19 for income on this T4?