Failure to Flash ESP01 with ESP8266 chip

[nsadding]

Hi.
Firstly, thank you for your work on this project and for sharing it in the public domain. I am always impressed by the selfless work that developers do and then share for no personal gain. It warms my heart and makes me feel that I need to do more to improve humanity!
Thank you! Thank you!.
I have been trying to upload your firmware to a esp8266EX based module and Ubuntu 20.04 PC, without success.
The firmware upload fails with the following terminal output.

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XXXXX$ ./flash.sh -t 1MB

==========================
Flashing ESP8266 firmware:

Flash type: 1MB
Flash mode: qio
Partition: 0
Address: 0x001000
Firmware: '../bin/upgrade/esp8266_AT_1_2.bin'
Erase Flash: 'yes'

esptool.py v4.0.1
Serial port /dev/ttyUSB0
Connecting....
Detecting chip type... Unsupported detection protocol, switching and trying again...
Connecting...
Detecting chip type... ESP8266
Chip is ESP8266EX
Features: WiFi
Crystal is 26MHz
MAC: b4:e6:2d:67:5b:ff
Uploading stub...
Running stub...
Stub running...
Erasing flash (this may take a while)...
Chip erase completed successfully in 2.5s
Staying in bootloader.

esptool.py v4.0.1
Serial port /dev/ttyUSB0
Connecting......................................

A fatal error occurred: Failed to connect to Espressif device: No serial data received.
For troubleshooting steps visit: https://docs.espressif.com/projects/esptool/en/latest/troubleshooting.html

Flash write failed!

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I am what I am doing wrong and hope that you can point me in the right direction.
Thanks and kind regards.
Neil