When trying to perform a Mass Erase of the chip, you get an error, e.g. UMI error. For mass erase to work, the CPU should be stopped. In case you have a multi-core device, the non-primary cores should be stopped as well.
Enable this option via the Hardware | <device> | Configure Device
dialog.
Follow how-to guide Disabling Flash Protection.
Open View | Debug | Session Explorer and check the status of CPU cores.