How to troubleshoot the Arduino Mini and Arduino usb2serial?
As always, it makes sense to check the parts in isolation, if possible.

Host system (Linux):


Tail the system log for errors
Check that ModemManager does not occupies the /dev/ttyACM0 device


USB2Serial:


Connect it via USB to the host without anything else connected -> the ON-LED should be on, the RX/TX LEDs should be off, the system log should display some udev progress messages (including vendor ids), the system should have a new serial device, e.g. /dev/ttyACM0
Open the serial port with a terminal (e.g. via minicom -D /dev/ttyACM0 [-b baudrate], disable local echo and type something -> nothing should be displayed
Connect the RX/TX pins to each other with a jumper cable -> typing in the terminal session should now result in rapid flashing of the TX/RX LEDs and the echoing of the text


Arduino Mini:


Connect just the minimum, e.g. just power from a 9V battery/5V power supply and the reset pin to +5 (with a 10KOhm resistor in series) -> the power-on led should turn on
The chance is high that the device comes with a pre-flashed blink examples that either uses pin 12 or 13 -> connect external LEDs to pin 12 and 13 (in series with 220 Ohm resistors) -> one LED blinks after power on
Hit the reset button on the mini -> any LED blinking should stop for a while and then continue


Arduino Mini with USB2Serial:


Make sure that both RX/TX pairs are crosslinked (like a nullmodem cable), i.e. RX &lt;-> TX and TX &lt;-> RX.
Follow the mini guide on arduino.cc or arduino.org it includes a picture of the cabling
Verify that the capacitor connected between the USB2Serial 'ext reset' pin and the Mini reset pin has a capacity of 100 nF, otherwise, the auto-reset might not work
In case the automatic reset does not work, press the on-board-reset-button just before the upload
In case the error persists, check for dodgy cables with a multi-meter


IDE:


Make sure that it works with 'easier' stuff, e.g. flashing to a Uno (the Uno is easier because the USB-2-Serial part is integrated)
Enable verbose logging during upload (in the preferences)
Update the IDE in case it is very old