I am running the latest Beta, but assume this issue has been around for a while… When I select Rebuild Tabe and attempt to change a CLOB to NVARCHAR2 in the script that is generated it keeps LOB Space definitions that cause errors. Here was the original table:
I then ran the Rebuild Table assistant and changed all the CLOBs to NVARCHAR2. Here is a portion of the generated script:
The workaround for me is to simply copy the script to an editor window and remove all the LOB statements, then it runs fine.