Hello @Phimbo, 

I hope this message finds you well.

Could you please specify which admin password your system is requesting? Is the prompt appearing outside the
Windows operating system, such as in the BIOS?

If the password is being requested within the OS and you do not have it, you will need to reinstall Windows.

How to Install Windows From a Dell ISO - Download the Windows 11 and Windows 10 ISO | Dell US

Just to add, system or BIOS password can be reset by jumper on motherboard.  Perform essential maintenance by replacing a new coin cell battery and reset CMOS at the same time.  Take precaution while working on system board.  Refer to owner's manual for instructions  https://dl.dell.com/manuals/all-products/esuprt_desktop/esuprtl_precision_workstation/precision-t3600_owner's%20manual_en-us.pdf