That PSU is absolutely fine., the (Recommended) PSU on AMD Graphics Cards essentially assumes a "Worst Case" Scenario.
The above system will draw:
Ryzen 5 1700 (4.0GHz OC) 145w (35w)
16GB (2x8GB / DDR4 3000MHz) 15w (15w)
RX Vega 56 (1.6GHz OC) 275w (18w)
512GB SATA-SSD 2.8w (2.5w)
2TB Barracuda SATA-HDD 5.6w (4.5w)
B450 Chipset 4.5w (4.5w)
----------------------------------------------------
Total: < 450w (80w)
A 550w 80+ PSU will Deliver a "Minimum" of 440w (Guaranteed) up to it's Rated Power Delivery.
I've listed in bracket the actual Draw during Boot / Idle / Desktop.
IF while playing Games, or engaging in Intensive Tasks your System simply Resets / Triggers a Surge Shutdown (i.e. Randomly Switches off) then the issue is that your PSU is unable to Deliver Enough Power.
I can't think of a single Scenario (today) where you wouldn't have enough Power to Boot / Install / Idle on the Windows or Linux Desktop., but if this was the case then the System would "Light Up" but nothing would be Active, with the BIOS giving 1 Short, 2 Long, 1 Short Beep (via the PC Speaker) to indicate that One or More of the Core Components is not "Present" during Boot,. which will typically be the GPU / PCI-E / USB Devices; of which only the lack of GPU will result in "Failure to Post"
No, instead if it appears like the Power is Intermittent (i.e. Cycle On / Off) means that there is a Short., either on the Motherboard or within the PSU; which in my experience is typically the PSU is just Dead-on-Arrival.
Of course as it's a "Semi-Modular" PSU, it's also possible that you've just not seated the Modular Connectors in the PSU properly and firmly as to create a Solid Connection... I'd recommend Reseating all of the Connections, as well as testing the Motherboard outside of the Case (I personally tend to do a Dry Run of all new Hardware via building it on-top of the Motherboard Box).
