Oracle Solution for Error ora-01150

ORA-01150: cannot prevent writes - file string has online backup set

What triggered the Error:

An attempt to make a tablespace read only or offline normal found that an online backup is still in progress. It will be necessary to write the file header to end the backup, but that would not be allowed if this command succeeded.

What should we do to fix it:

End the backup of the offending tablespace and retry this command.