Why does the UART not report an error for a data byte with wrong parity?
When you set UCPEN, the hardware sends (and expects to receive) an additional bit:



If the actual data bits D0…D7 already have the correct parity, the PA bit is transmitted as zero. If the D0…D7 parity is wrong, the PA bit is set to one, so that the parity of all bits together comes out right.

The parity checking mechanism never restricts what data bytes you can send; its only purpose is to detect (some) transmission errors.



As long as you are using the same eUSCI module with the same configuration, it is not possible to force a parity error.
What you'd have to do is to use two different UARTs, configure the sender for eight data bits without parity, and the receiver for seven data bits with parity. The sender's D7 bit can then be set to a value that does not match the receiver's expected parity.