If a day won't take an edit, it's almost always one of these four things.
The date is outside your planbook's range
This is the common one. Dates shown in black rather than as editable boxes fall before your planbook's Start Date or after its End Date.
Click Planbooks, then Edit next to your planbook, and check the two date fields. Widening the range makes those days editable — and any plans that were already there come back into view. See Planbook basic options.
The day is marked as an off day
Off days aren't teaching days, so they don't take plans. Click the cog next to the day name and choose Remove Off Day if it was marked in error — see Set up off days, holidays, and breaks.
The planbook is archived
Archived planbooks are readable but not editable. Click Planbooks, then Show Archived Planbooks, and choose Activate on the one you want to work in. See Archive a planbook.
You're viewing a shared planbook
If you reached the planbook through a share link or as an administrator, you're looking at somebody else's plans in read-only form. Only the owner — and a teacher they've explicitly set up to collaborate — can edit.
Check the planbook name at the top. If it isn't one of yours, click Planbooks to get back to your own.
Still stuck? Contact support with the planbook name and the date you're trying to edit.