System Storage = Operating System currently running on your device (no way to change/modify that storage occupation).
System Data Storage = This category is mostly filled with cache and system files created by Safari, the Mail app, other apps and iOS/iPadOS itself, and that space is automatically managed by the operating system and you can’t interact with it. (5-15GB is a "normal" occupation).
Unfortunately your iPad model cannot support iPadOS 13.6.1, and you can't perform the steps provided in this support article--> Clear Other storage on your iPhone or iPad - Apple Support
Some benefits to reduce that space can be also obtained creating a full backup of your device, then restoring it to factory settings and, in the end, setting it up as new, as per these instructions: Restore your iPhone, iPad, or iPod to factory settings – Apple Support
When done, restore data from your previously made backup, then restart your device and check your device storage again.
If the issue persists, contact Apple Support directly, using this link: Get Support
Regards
Giulio