  In the sense of distributions, the derivative of the Heaviside unit step
function $H(t)$ is a generalized Dirac-$\delta$ distribution. If the velocity
$V(t)$ of a flat plate is impulsive, as $V(t)=H(t)$ (i.e., it is suddenly set
into motion with unit velocity at $t=0^+$), then its acceleration is
$V'(t)=\delta(t)$. The Dirac-$\delta$ distribution has no point values.
However, when the Dirac-$\delta$ is the forcing term of an ODE (in $t$), it
contributes to the solution. The recently published paper [Chaos Solitons
Fractals 117 (2018) 68] incorrectly treats the Dirac-$\delta$ function as being
identically 0. This Comment analyzes the source of this error, and provides
guidance on how to correct it (based on the established literature). The
mathematical error identified is in addition to some issues about rheological
models with fractional derivatives, which are also noted. That is to say,
whether or not an "Atangana--Baleanu fractional derivative" is used in [Chaos
Solitons Fractals 117 (2018) 68], the solution to Stokes' first problem
provided therein is not correct.
