For your security, the Dataplicity client uses an unprivileged service account. If you want to use 'sudo' from the Dataplicity terminal, for example to allow you to install programmes or manage your device, you must first switch to a more privileged user account and enter the password for that account.
Here's the video howto
Prefer the tutorial? Here goes...
On a Pi, the default 'pi' account is suitable. To switch to the 'pi' user, generally indicated by the pi@raspberrypi / $ prompt, just type:
You will be asked to enter the 'pi' account password which, unless you have changed it, is raspberry.
When your screen looks like this you're ready to go
From there you can 'sudo' as normal:
sudo cat /etc/shadowTada!