Before writing anything, I want you to inform that 'cancel' a rebuild task is not something nice to do and could land you up in a bad scenario.

Please try to make sure the following conditions are met:

1) Make sure you have sufficient amount of transaction log space and do not let 'autogrowth' parameter increase the size and give it that much in advance.

2) Make sure not to perform this when you have a lot of DMLs going in your tables, at times in nights there are a lot of imports running so keep an eye there.

3) TempDB should have enough space and again, dont let autogrow happen as much as possible.

To getting quicker results you should remember that 'Rebuild Index' task is considered as a bulk operation, so you might want to change the recovery model to 'Bulk-logged' for minimal logging and once you are done change the recovery model to full and issue a fresh full backup to keep the chain intact.

Let me know how it goes. All the best!!!

Chandan