You’re drag and dropping an existing booking from one resource's calendar to another, but you're unable to.
The resource isn't assigned to the service
Nine times out of ten, it’s because that resource is not assigned to the service. You can tell this because the calendar of the other resource will change colour to grey:
If you try to drop a booking on a resource who is this grey colour, you'll receive an error.
If you expect that resource to be assigned to the service, here's a few things to check:
Do you have any similarly named services? Ensure you’re interacting with the right service. e.g. Payroll Meeting (existing customers category) vs. Payroll Meeting (new customers category).
Check the services assigned to that resource: go to Setup > Manage resources, click view info, click edit resource then check and edit the assigned services.
The resource isn't available at that time
Check that the resource is available. The working hours will reflect in white. If the resource isn't available according to the working hours, you can add an exception to their working hours so they're available at that time. There's a guide to this here.
Alternatively, it could be that the resource is busy with other bookings or external calendar events. Appointedd does not allow double bookings so you can't move a booking to a resource when that resource has another booking.
If you're still unable to make the booking, check out this guide which takes you through all of the factors which feed into your availability.