I have a DTK Mini machine and I want to enter safe mode, but I click cmd + R and wait for a while. The system always starts normally without entering safe mode. What should I do?

Hi, I applied for and purchased an Apple DTK device, and installed and upgraded the system to Big Sur Beta6 (20A5364e). I found that after rebooting and pressing cmd + R, I cannot enter the recovery mode. Always start normally. Can you tell me how do I do?