Windows keeps its clock accurate by synchronising with an internet time server, and this behaviour is controlled from Settings → Time & Language → Date & Time. To understand the answer, it helps to know how automatic time-keeping works.

When Set time automatically is turned ON, the operating system periodically contacts a network time service (such as time.windows.com) using the Network Time Protocol (NTP) and overwrites the local clock with the server's value. Because Windows is actively managing the clock in this mode, the manual Change button under "Set the date and time manually" is greyed out (disabled) — any value you typed would simply be overwritten at the next sync.

Therefore, Set time automatically must be disabled (turned OFF) first. Only then does Windows hand control of the clock back to the user and enable the Change option, allowing you to enter a custom date and time.

The remaining choices are unrelated to setting the clock value:

• Night Light only reduces blue light on the display in the evening; it has nothing to do with the clock.
• Enable multiple clocks merely adds extra clocks for other time zones to the taskbar flyout; it does not lock or unlock manual editing.
• Set time zone globally (or the automatic time-zone setting) affects which zone offset is applied, not whether the date and time can be entered by hand.