Each requestable activity has been set up uniquely. Therefore there are various reasons to why certain types of activities are not requestable for specific day in time, while others are. For example:
Blackout period during peak season, No absences allowed because of the contact volume expected.
Too early to place request, The date range has not yet been unlocked by the administrator as it is too far in advance.
Already fully booked by others, It is a popular date and it has been blocked to secure the right coverage on the floor.
Too late to place request, there was a deadline for that specific date to anticipate popular periods well in advance.
Depending on the applicable reason decide to choose another day to request time off, or safe the days for another moment.