This usually happens when an email provider has a default name for a folder that is different from the default assumed by Thunderbird. Normally, only one is being used. You might check online to see which is the default by your email provider and adjust the default for Thunderbird to use. My suggestion is to use the email provider's, as they generally don't allow their folders to be deleted, whereas the default of Thunderbird's can typically be deleted. For example, my email provider defines junk folder as 'junk mail', whereas TB uses 'junk.'