ISPF transaction log not available

Sometimes the log is not available for browsing. This can occur when:

The log data set is empty.
The log data set was not created for this session because 0 was entered in the "Primary pages" field on the Log Set Defaults and List Set Defaults pop-ups, which can be reached by selecting Option 0 from the ISPF Primary Option Menu and then selecting the Log Data set defaults and List Data set defaults choices from the Log/List pull-down.
No data has been written to the log during this session, and although the log data set exists and is not empty, you did not end the last ISPF session normally; for example, an abend can have ended the session. You can browse the log if you take an action that causes a log entry to be written.
The log data set was previously allocated with a disposition of OLD. It must be allocated with a disposition of MOD.
The log data set has been previously allocated to SYSOUT.