Windows keeps the system clock synchronised with an external reference so that the date, time, and time zone shown to the user always remain accurate without manual intervention. This is done through the Settings → Time & Language → Date & Time page.

How the automatic time toggle works:

• The Set time automatically toggle, when ON, makes Windows periodically contact an internet time server (commonly time.windows.com, an NTP — Network Time Protocol — server) and silently correct the local clock to match it.
• Because the operating system is actively overwriting the clock value in the background, Microsoft disables manual editing while this feature is active — if a user tried to set a custom date/time while automatic sync is on, the very next sync would simply revert it. To avoid this confusing behaviour, Windows greys out the "Change" button under manual date/time settings whenever automatic syncing is enabled.
• Therefore, to manually type in a specific date and time, the user must first switch Set time automatically to OFF. Once disabled, the "Change" button becomes clickable and the user can input the desired date and time values directly.
• A related toggle, Set time zone automatically, works similarly for the time zone (using location services), and may also need to be turned off if the user wants to pick the time zone manually — but the question specifically concerns manually setting the date and time, which is governed by the "Set time automatically" toggle.

Why the other options are irrelevant:

• Night Light is a display feature that shifts the screen to warmer colour tones in the evening to reduce eye strain and has nothing to do with the system clock.
• Enable multiple clocks lets a user add extra clocks (for different time zones) to the taskbar clock flyout for reference purposes; it does not control or block manual date/time editing.
• Toggling the time zone globally (i.e., how the time zone is applied across the system) is a distinct setting from the date/time value itself and is not the toggle that must be disabled to unlock manual date/time entry.