Update!
I have finally resolved this upload issue:
1 - The SAMD21 WIKI notes the following:
While this is the solution, I have discovered that this is a bit tricky in that ‘flicker’ can be difficult to decipher. As noted above I think I had previously (successfully) uploaded a Blink sketch because the yellow LED was in fact ‘blinking’ and perhaps not ‘flickering’;
2 - Patience helps because shorting the RST pins can be a bit frustrating. Finding the correct ‘tool’ to short the pins helps. Here is my rudimentary tool that gets the job done:
3 - Lastly it takes a bit of practice but the reward of being able to successfully upload your code to the SAMD21 is well worth the effort and time invested.
IMO - each of the SEEED MCU’s are amazing in every way. I have a tray of each of these SEEED MCU’s and am always astonished at what such a small MCU can do.
Thanks.