If you cannot boot into safe mode or the error message persists, you can use a recovery disk or a live CD to troubleshoot the error message on OS startup. A recovery disk or a live CD is a removable media that contains a bootable OS or a set of tools designed to help repair or recover your system. To use one, you need to insert it into your computer and change the boot order in the BIOS or the boot menu. Examples include Windows installation media or Windows Recovery Environment (WinRE), Linux installation media or Linux live CD, and macOS Recovery or macOS installation media. With these disks, you can perform various tasks such as repairing or rebuilding the boot sector, the master boot record (MBR), or the boot configuration data (BCD) using commands such as bootrec, fixboot, fixmbr, or bcdedit; checking and fixing any disk errors or bad sectors using commands such as chkdsk, fsck, or diskutil; resizing, deleting, or creating partitions using commands such as diskpart, gparted, or diskutil; and recovering any lost or deleted data using commands such as photorec, testdisk, or ddrescue. This should help you troubleshoot the error message on OS startup and inform and reassure stakeholders.