The warning regarding maximum shipping length is controlled in Job Properties. It is useful for locations that stock a limited product length and want to ensure that designers are not using lengths that cannot be shipped.
Here is a typical warning. The design includes AJS joists that are only available in lengths of 48 feet or less.
To change this setting, go to Job Properties > Job Settings > Design > Structure Check Preferences: Shipping Lengths. The various category types may be given a maximum shipping length or you may turn off the check entirely.