You have about four choices to fix your problem, as mentioned by neon Link in his comment:
1) Take the motherboard from where you purchased it and have them update the BIOS to work with your Ryzen CPU/APU. Or take it to a computer repair center. But that will be expensive.
2) Try finding a older Ryzen CPU/APU that will work on the Motherboard BIOS Version and install it. Boot up into BIOS and update it to the correct BIOS version for your new Ryzen CPU/APU
3) Turn in a AMD RMA REQUEST FORM Online and have AMD Support mail you a BOOT KIT free of charge. Copied from previous link: https://support.amd.com/en-us/warranty/rma
4) I heard some of the Latest Motherboards now are able to update the BIOS without having a CPU/APU installed. You need to check with your Motherboard's Manual if your motherboard has this new feature in it. EDIT: Did check. Your motherboard doesn't have this feature.
According to Asus Support the minimum BIOS Version that works with the Ryzen 2600 is BIOS VERSION 3803, the latest version for your Motherboard is 4011 : PRIME B350-PLUS Driver & Tools | Motherboards | ASUS USA