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

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wf121:

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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

Notes:

1. Refer to Table 2: Multifunction pad descriptions and section 5.2 Serial Port in the datasheet for UART pin assignment


2. In datasheet, the UART port number starts from 1. In configuration user guide, UART channel number starts from 0.

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.