Ensure your UART adapter's TX is connected to the board's RX, and GND is securely connected to both.
Disconnect any other external components (like WiFi dongles or USB drives) that might be sharing the power rail or communication bus during the flash process.
is a cross-line/null-modem type, or are you using a standard straight cable?
To understand the fix, you have to understand the message.
You need to see if Windows actually sees the device.
Here’s a useful technical review of the error message: