99-platformio-udev.rules is out of date
This is a recent issue, which is being looked into. It appears to be entirely cosmetic… I have it myself and it doesn’t seem to be stopping anything from working. As long as you have installed the udev rules, restarted udev so they are being applied, and have ensured your username is a member of the relevant groups(dialout, plugdev), you can ignore it. i.e. you’ve followed the instructions in the FAQ on this warning message.
After finding out I needed a paid membership to use the debugging features I subscribed.
Not to niggle, but did you try the 30 day trial first?
Best place to start when looking for info on a particular board is the documentation page for that board. i.e. go to the Boards page, search for your board, and then click on it to get to it’s specific documentation page. Like the one for the Teensy 3.1/3.2. Similar thing for debugging… i.e. seeing what specific configuration options are needed for the debugger. In this case, it’s just a matter of copying the debug_tool = jlink line. And the GDB Server will be used because it allows for both local and remote debugging capabilities, among other things.
For the Teensy 3.5 (as I don’t have a teensy3.0/3.1 anymore)… it seems like there are some issues which run deeper than just PlatformIO… a simple configuration such as
[env:teensy35]
platform = teensy
board = teensy35
framework = arduino
upload_protocol = teensy-cli
debug_tool = jlink
(I’m using a Teensy 3.5, and switched to the CLI uploader, instead of the default GUI, simply so I don’t have something else popping up over the screen - it doesn’t change any other behaviours).
… and the following code …
#include <Arduino.h>
void setup() {
pinMode(LED_BUILTIN,OUTPUT);
Serial.begin(9600);
}
void loop() {
digitalWrite(LED_BUILTIN,!digitalRead(LED_BUILTIN));
Serial.print("Elapsed millis ");
Serial.println(millis());
delay(1000);
}
… it upload fine - PlatformIO auto detects the Teensy 3.5 programming port, and works fine. If I try to monitor that port… it also auto detects it… and either fails with could not open port '/dev/ttyACM0': [Errno 16] could not open port /dev/ttyACM0: [Errno 16] Device or resource busy: '/dev/ttyACM0' or opens the serial monitor, but there’s no output.
Thinking it could be a gremlin in the platformio serial monitor, I opened minicom instead (minicom -D /dev/ttyACM0 -b 9600)… and no cigar… either the same error, or nothing. Checked permissions… /dev/ttyACM0 is in the dialout group, so shouldn’t be a problem… unplugged the teensy whilst minicom was running… plugged it back in… immediately got output… for all of five seconds.
Will have to dig deeper… and reinstall Teensyduino… to see if there is something wrong with the teensy stuff, or with the compiled platformio code.
However, PlatformIO is not always ‘easy’ … especially when you’re getting started… partly because it is supporting all the different OSs and processor frameworks, etc and trying to bring it all under one roof… but once you get over the learning curve of searching the docs for stuff - either libraries, board configurations or settings for stuff, and get comfortable with that platformio.ini I think you’ll like it. And don’t be afraid to post on the forum! 
