When a MS SQL database back-up is executed, it could throw the following error message:
LOG is not allowed while the recovery model is SIMPLE. Use BACKUP
DATABASE or change the recovery model using ALTER DATABASE.
This means there is a Transaction Log Backupschedule present, while your database mode does not support this.
You can solve this issue by removing the Transaction Log Backupschedule, so it is no longer executed. You will still have your full- or incremental backup though.
In the screenshot shown above, you can find a default back-up schedule for a MS SQL database. You can safely remove the first schedule; Transaction Log Backupschedule. This is the backu-up set that causes the errors.
The other back-up sets will continue to work (and have always worked), on the pre-defined times. The error message shown was only for the Transaction Log Back-up, the other back-ups were executed without errors.