Board won't power on

What PSU are you using? Do you have the system inside a case? If you have not
already done so, try removing the board from the case and connecting it up on top
of it's retail box with just 1 stick of RAM in slot B1 (furthest from socket), the
CPU, GPU, display output, keyboard and power connected.

The Ryzen 5 2600 does not have integrated graphics so you will need a discrete GPU.

Never use no name PSUs, better to get a low wattage branded PSU (at least 80+
certified) than any no name garbage. A 400w 80+ bronze PSU will be better than
a 600w no name, non certified piece of trash. Most of the no name brands lie in their
ratings, I have seen units sold claiming to be 750w but in fact have cheap 250w internals.

The PSU is the most critical part of your system, if it blows up it will very
often take out other components with it, particularly uncertified models that
have little to no protections in place to prevent it. The easy way to tell if
a PSU is garbage is that it will not be 80+ certified. I see you are in Greece,
take a look at FSP, DeepCool, Segotep (Segotep are not great but better than no name) and Cooler Master.

I know it's tempting to pick up a 10-20 euro PSU instead of getting a good one
but it will always come back to bite you.

never skimp on the power supply .. most important part of your build.

Hopefully you haven't damaged any of the electronic components with that, what appears to be, a faulty PSU.

Hello again!
I just try my new PSU 430w 80+ bronze.
Still nothing...