The SI unit of average velocity is:

Hint:Average velocity is defined as : the rate of change of total displacement in total time .
Whenever the word rate of change comes in a definition then it means that quantity which is being changed is divided by time .Like in the question displacement is divided by time. Similarly for speed rate of change of distance ,rate of change of velocity for acceleration .

Formula used :$\dfrac{{total displacement}}{{total time}}$

Complete step by step solution:
SI Unit of displacement is $m$ (metre)
SI unit of time is $\operatorname{s} $ (second)
Therefore , when total displacement is divided by total time it gives the average velocity but the SI units will remain unchanged even when displacement is added and time is added.
So ,$\dfrac{m}{\operatorname{s} }$ is the unit of average velocity .
Or we can write as $m{s^{ - 1}}$ (shaving superscript minus 1 because when s goes up in multiplication with m ,power of s becomes negative).
Average velocity can also be found out by addition of all the average velocities divided by total number of velocities but the SI unit will remain the same irrespective of formula used

Note:Velocity and displacement are vector quantities .By vector we mean that a quantity that has both direction and magnitude.
-Like, a train is moving at 80$m{s^{ - 1}}$ in the north direction .Direction mentioned here is north and magnitude of velocity is 80$m{s^{ - 1}}$.
-While scalar quantities are those which have only magnitude and no direction. For example , speed ,distance. Although speed too has the same SI unit but it has no direction .