I assume that this activity was assigned to all your students (and not only the specific students or sections that you have already graded). The only time I have seen the gradebook not allow entering a score is for a student who was not assigned to the activity.
Since you are not seeing a subtotal for the group, you might also check the assignment group weighting from the settings at the top of the assignment tab. If your groups are weighted, any weight, including a zero, should create a subtotal. New groups usually have a default weight of zero when you use weighted groups, but maybe something went wrong creating it.
In speedgrader, I have only seen names greyed out when there is neither a submission nor an existing score, and even there I can type in a score if necessary. The only time the box to enter a grade has been greyed out is on quizzes with an existing submission. You need to enter a score for each question and then click update at the bottom of the screen. There is a delay/need for repetition with the update button right now, but it updates the greyed out score.
I think K-12 schools sometimes assign grading periods to groups, which could block grade entry if turned on. I do not have access to that type of setting, but if it is available to you, that is worth checking also.
Canvas support should be able to help you even if none of those ideas help to find the problem
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