I cannot flash anything to my WF121 following instruction found in WF121 Configuration Guide v19 so I found your post and tried to do what you said in your PDF but MPLAB IDE seems to have changed and I cannot found the Programmer menu.
My Pickit 3 programmer JAM version is 01.31.20-00 which is more recent than on your PDF but should it be a problem?
WF121 default factory setting:
The default factory firmware is in \wif\example\wf121\ directory in the SDK.
In \wif\example\ directory, there is readme.txt file, which gives a brief description for the examples in the SDK.
In readme.txt, you can find
Default firmware used in the production.
This example configures the BGAPI to available over UART1 (note: it is UART channel 1 and see below remarks)
UART settings: 115200,8n1 and HW flow control enabled
UART channel 0 = UART 1 in datasheet
UART channel 1 = UART 2 in datasheet
3. In the project.xml file (wf121 example), you can see the following lines for UART configurations:
<uart channel="0" baud="115200" api="false" />
<uart channel="1" baud="115200" api="true" handshake="True" />
Uart channel = “0” means UART 1 in datasheet. This UART is disabled for BGAPI but you can this UART port for application data (e.g. TCP data) or debug data.
Uart channel = “1” means UART 2 in datasheet. This UART is enabled for BGAPI for external host MCU. This is also the bootloader interface for DFU firmware download.