The reason you are seeing the above error is because a password has been set on one or all of your distribution points, you will need to login to the SCCM console in order to remove or change this password.
Open your SCCM console and navigate to the Administration page, once there click on Distribution Points.
Right click on a Distribution Point and select Properties.
A window will open and you will see multiple tabs along the top, you will need to select PXE which is the second tab.
Untick the box that says “Require a password when computers use PXE” or change the password.
Reboot the machine you were trying to build and you will be good to go!