When exporting dashboards from on-premise to online you need to allow that dashboard export for a later version of Business Analyze.
(This also applies if you are trying to export from an older version to a new version on a new server for example)
Online enviroment is currently running on a newer version than availible for on-premise.
If you did not create a package to export read this first:
Here is how you allow the package for other versions:
Once you have chosen to export the package you will be promted with this window:
Click f12 to bring up devtools for your browser and choose "Select an image in the page to inspect it"
Click this dropdown:
Remove display: none as shown below
You can now choose the supported platform