How can I disable Auto-Lock on iPadOS 13?
I faced this same issue on an iPhone few months ago.
Why this happens ?
When you link a device to a corporate email address, the Auto-Lock setting can sometimes be restricted by the corporation for security reasons, so that people cannot access the device if the user unintentionally forget to lock it.
Depending of the company preferences and device concerned — it may vary between 30 seconds and Never as maximum time of inactivity on the device.
Example
If the company set the maximum inactivity setting to 15 minutes : the Never option will not be shown (as it is the case in the photo you posted).