When you program the application firmware for the nRF9160 System in Package (SiP) of a Nordic Thingy:91, build the application for the nRF9160 SiP. Before programming it, put the Nordic Thingy:91 in the application serial recovery mode. You can activate this mode by holding in the SW3 button while powering Nordic Thingy:91 off and on.
If nrftil device fails with a timeout error during the erase step, use the same procedure and put it into application serial recovery mode using the SW3 button.
Note
If you press the wrong button while programming, you can put the device into the application serial recovery mode for the nRF52840 System on Chip (SoC). This makes it unusable. The device no longer appears in nrfutil device list. You must then reprogram the nRF52 connectivity bridge firmware using the external debug probe of the nRF9160 Development Kit (DK). See programming instructions in Updating the Thingy:91 firmware using Programmer.