If your clients can't modify and/or cancel their appointments even though the "Cancellation and Modification by client" setting is enabled, it may due to the following reasons:
It is past the cancellation and modification allowance delay. For example, if your delay is set to "24 hours", and it is currently 10 hours before the scheduled appointment time, no modification or cancellation will be allowed. To remedy this, you can edit your allowance delay, or simply keep it as is and edit/cancel the appointment directly in the Calendar tab on behalf of the client.
You believe that your business' setting applies, whereas in reality it is your service's.
If the "Cancellation and Modification by client" settings are enabled at both the service and business level, the service settings have priority. Consider this example:
Your business setting is set to: "Allow both (modify and cancel)"
Your service setting is set to: "Allow Modify"
Because of the rule stated above, the system will apply the service's setting, meaning that clients will only be able to modify, not cancel. If this is the case, either disable the service setting completely (as a result, the business' setting will apply) or edit the service's setting to match your desired outcome.