From Lenovo website: “For some models, instead of pressing F1, continuously press Enter during powering on until a Startup Interrupt Menu displays.” Which means to me that you need to “turn-off” this Lenovo Thinkpad T550 by restarting it from WinOS (not by power-off as this will lock your HDD) and start to continuously press F1 after you see that T550 laptop is off and shows some initial power-on light (sign of life).
@knurtsj, page 103 is showing you that you need to set BIOS there to Legacy Only.
And if you want to completely remove OS that already installed there (after you backup what necessary) we will help you to install PureOS there? Please take our good will to help you to get familiar with the Linux world. Please change boot order inside of BIOS as well after finding if pressing F1 or F2 or F7 helps to get into BIOS, your first move toward … I’m not familiar with but still suppose that T550 hardware is good place to start (do not give up with your positive idea, also you do not need to rush yourself or us but everything looks fine to me to indeed have a new start).
EDIT: Page 78: “Restart the computer. When the logo screen is displayed, press F1 to start the ThinkPad Setup program.”