Thanks for reaching out and for your interest in CryptPad!
Sadly, Tor Browser block this functionality by default. To unlock it you have to switch the security level of the browser from Safe to Standard. Please see the screenshots below:
By clicking "Change..." you are taken into the general settings of Tor Browser, you can then switch to Standard level and reload your CryptPad tab.
Now the export functionality should be fully working.
Hope this helps!