Why can a pace or a footstep not be used as a standard unit of length?

A pace or a footstep cannot be a standard unit of length:

A pace or a footstep cannot be used as a standard unit of length due to differences in the size of body parts.  A footstep or a pace can be used as our own units for convenience. But internationally such types of units for measurement are not allowed.

SI units of measurement:

Some standards have been set to measure physical quantities. These are called SI units. SI units are internationally accepted units. It stands for 'System International'.

Example of SI units of measurement:

Mass can be measured in grams, kilograms, tons, etc. But the SI unit of mass is kg. Similarly, the length can be measured in meters, kilometers, miles, etc. But its SI unit is a meter. 

SI units of measurement for physical quantities:

In 1939, the CCE recommended the adoption of Internation units and got approval. The SI units are sometimes also known as MKS units $(meter, kilogram, and second)$. 

SI units for measuring length, mass, and time are as follows:

Name of the Physical quantity	SI unit	SI unit symbol
Length$(L)$	Meter	m
Mass$(M)$	Kilogram	kg
Time$(T)$	Second	s