Error 0x80310087

Value: -2144272249 | 0x80310087 | 2150695047

What does it mean ?

FVE_E_POLICY_CONFLICT_RO_AND_STARTUP_KEY_REQUIRED  
BitLocker Encryption cannot be applied to this drive because of conflicting Group Policy settings. When write access to drives not protected by BitLocker is denied, the use of a USB startup key cannot be required. Please have your system administrator resolve these policy conflicts before attempting to enable BitLocker.
Value: 135 | 0x0087 | 0b0000000010000111

Where does it come from ?

FACILITY_FVE  
Full volume encryption
Value: 49 | 0x031 | 0b00110001

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