The first place to look in is the Management Console -> Events in SaaS, to see if the engine logged a failure with more details. The next place is the SaaS tenant logs, which can be reviewed by Support. You could submit a Support case with the Tenant ID and User ID, to have the back-end logs checked. If the failure might be network-related, you can monitor this with developer tools in the browser, and collect a HAR file if needed.
A common reason of app upload failure is due to a capacity limit. If your .qvf file size on-disk is over 1 GB and you only have Standard capacity, this is likely. Or, you might be hitting a storage limit for apps + data in your tenant. Finally, the app in-memory size must be under the documented limits, which is checked during upload using the file metadata. SaaS capacities vary between Qlik Sense Business, Qlik Sense Enterprise SaaS, and Expanded apps or Dedicated capacity. Further capacity details:
https://help.qlik.com/en-US/cloud-services/Subsystems/Hub/Content/Sense_Hub/Introduction/sense-capac...
