Why a fix obtained from the GPS service cannot be plotted directly on to a navigational chart?

Most of the admiralty chart referred to a local data but position obtained from the GPS system refers to WGS 84 data. Charts published since 1951, carry a note indicating the amount of the shift, according to the WGS 84 data.

If navigational charts not contained WGS 84 chart datum, then fix obtained from the GPS receiver cannot be plotted directly or these charts.

In other words it can be explained as:

For the GPS user, concerns are focused on the need to ensure that latitude/longitude datum shifts are made when plotting GPS-derived positions on a chart with a different datum. This problem is exacerbated in the near term because National Imagery and Mapping Agency (NIMA) nautical charts currently are compiled on different datums. Full conversion to the standard datum, World Geodetic System 1984 (WGS 84), for over 4,000 NIMA nautical charts is not expected until the year 2002. Accordingly, the user could be required to apply several different datum shifts to the acquired GPS position, which is based on WGS 84, as the ship’s track is progressively plotted on subsequent charts with possibly different datums.