Above 4G decoding, Resizeable BAR and CSM disable are the 3 BIOS settings to look for. Make sure all are set, save the BIOS settings and exit and boot to Windows, SAM should enable now.
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT