Sounds like your database is in SIMPLE recovery mode. Databases in SIMPLE recovery mode can not have there transaction log backed up, as the transaction log is truncated each time the database flushes the log changes to disk. In order to backup the transaction log for this database you will need to change the database recovery model from SIMPLE to BULK LOGGED of FULL.
Do you know the recovery model of the database in question?
To be able to backup the database’s transaction log (allowing point in time recovery) it must be in full recovery mode.
I expect it may be in simple mode (which will generate an error if you try and backup the log) and the maintenance plan’s validation is stopping you from a selecting an incorrect configuration.
If it is has a full recovery model please post back.