There are two potential explanations for this:
The Bookings has already started and the user's role is Researcher, Student, or Guest
If the user has a Researcher, Student, or Guest role, they are not able to cancel a booking once it has started. Now, if the user has created a continuous booking, done by dragging the booking slot over multiple days/weeks, the system recognizes it as one booking and the user will not be able to cancel the booking. Please note, on the lab dashboard the booking might appear to be broken up into individual bookings, however it may still be one continuous booking if the user created it by dragging the booking across multiple days
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Additionally, if you go to Equipment>Bookings page, you can see the frequency of the booking (single or recurring). For recurring bookings, even though the user might not be able to cancel the current one, she will still be able to cancel any future bookings.The user's settings do not allow cancelling bookings.
The access rules in either the user's individual settings, or the access group that they have been assigned the equipment from does not allow cancelling bookings.
Please find where the user has been assigned access to this instrument that they are trying to cancel their booking for. This could either be assigned to them in their individual user rules or via an access group. In the relevant settings, if you wish to allow the user to cancel their own bookings then turn this option to On.