  Scintillation of compact radio sources results from the interference between
images caused by multipath propagation, and probes the intervening scattering
plasma and the velocities of the emitting source and scattering screen. In
FRB20201124A, a repeating fast radio burst (FRB) which entered a period of
extreme activity, we obtained many burst detections in observations at the
upgraded Giant Metrewave Radio Telescope (uGMRT) and the Effelsberg 100\,m
Radio Telescope. Bursts nearby in time show similar scintillation patterns, and
we measure a scintillation timescale of $14.3\pm1.2$\,min and $7\pm2$\,min at
Effelsberg (1370\,MHz) and uGMRT (650\,MHz), respectively, by correlating burst
pair spectra. The scintillation bandwidth scaled to 1\,GHz is $0.5\pm0.1$\,MHz,
and the inferred scintillation velocity at Effelsberg is
$V_{\mathrm{ISS}}\approx (59\pm7) \sqrt{d_{l}/2\,\rm{kpc}}~{\rm km~s}^{-1}$,
higher than Earth's velocity for any screen beyond a lens distance of $d_{l}
\gtrsim 400\,$pc. From the measured scintillation bandwidth, FRB20201124A has
comparatively lower scattering than nearby pulsars, and is underscattered by a
factor of $\sim 30$ or $\sim 1200$ compared to the NE2001 and YMW16 model
predictions respectively. This underscattering, together with the measured
scintillation velocity are consistent with a scattering screen more nearby the
Earth at $d_{l} \sim 400\,$pc, rather than at the 2\,kpc spiral arm which
NE2001 predicts to be the dominant source of scattering. With future
measurements, the distance, geometry, and velocity of the scattering screen
could be obtained through modelling of the annual variation in $V_{\rm ISS}$,
or through inter-station time delays or interferometric observations.
Scintillation/scattering measurements of FRBs could help improve Galactic
electron density models, particularly in the Galactic halo or at high Galactic
latitudes.
