Question New pc build, Bootmgr is missing.

Jul 24, 2019
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Hello everyone, normally I'm a very solitary, very I'm going to solve it myself kind of guy. I've been beating my head against the wall for a few days now and decided I either need to ask for help or just have $1000+ of pc parts sitting around doing nothing.
So the problem is this, I keep getting to where it should install the os (linux mint) but keeps saying bootmgr is missing and insert boot media and press a key.
Now I've made sure to try putting the iso on multiple disks and flashdrives. I've also swapped the boot order around for each one and even tried booting directly from the disks and flashdrives, still get the same thing. Ah also the video card isn't installed yet.
Parts list
Asus ROG STRIX B450-F GAMING ATX AM4 Motherboard
AMD Ryzen 5 2400G 3.6 GHz Quad-Core Processor used
Thermaltake Water 3.0 Ultimate 99 CFM Liquid CPU Cooler
Corsair Vengeance RGB Pro 32 GB (2 x 16 GB) DDR4-3200 Memory used
Seagate Barracuda 3 TB 3.5" 7200RPM Internal Hard Drive
Kingston A400 240 GB 2.5" Solid State Drive
MSI Radeon RX 580 8 GB ARMOR OC Video Card
Thermaltake Tower 900 Snow Edition ATX Full Tower Case
EVGA SuperNOVA G2 850 W 80+ Gold Certified Fully Modular ATX Power Supply used
Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer used
Gigabyte GC-WB867D-I PCIe x1 802.11a/b/g/n/ac Wi-Fi Adapter


The used parts are from ebay. Oh also have a motherboard beeper thing and all it does is one beep as it's booting up which as far as I know from lookin at the manual means everything's okay. The board also has a similar light system which also says everything is okay.
No idea what's wrong, so any and all help is appreciated. This is my second build, but still don't have a whole lot of experience with this.