If your ISP has in their PHP configurations a memory limit below 256 megabytes probably will to receive this error. We have 2 ways to solve this issue:
1) To get jUpgradePro working you must to download the vendor.zip package available in the Downloads section and decompress it into the jUpgradePro component folder:
JOOMLA_ROOT/administrator/components/com_jupgradepro/vendor
2) If you have access to your OS command line (Linux/Mac/Windows) and you have Composer installed you can go to jUpgradePro path and execute the following command:
$ cd JOOMLA_ROOT/administrator/components/com_jupgradepro/
$ composer update