Just like any hard drive, your Garmin can get filled with files you don't need anymore. Here are a few suggestions on how to get things working.

1. Reduce Dataset Options. IF you are using the defaults (all locations, all POI alerts) you need approximately 45MB available on your Garmin device. You can reduce the size of your dataset by selecting only the locations (states) and POI Alerts you want. Not really a long term fix, but it may get you going in the short term.
2. Remove existing Custom POI's. You may have downloaded someone elses dataset in the past. Walmart.gpi, HotDogStands.gpi, etc. Whatever it is, you many want to manually review and remove them to make more space. Here's how.
   
   Usually your .gpi files are in the \Garmin\POI folder on the GPS. Attach your GPS to your computer via the USB cable - your computer will recognize it as an external drive / device. Take a look at the contents of your GPS with file explorer (PC) or finder (mac) and visit that directory. Delete any large POI files in that directory you don't use anymore (POI.gpi is a likely candidate) and use file explorer / finder to check the available space on your device. If you have enough space, then detach your GPS, turn it off / on for good luck, and try retrieving your dataset again.
3. Still out of space? Wow, you must have an older unit - Garmin was a bit stingy in the space department. Consider this - you can remove the language files you don't need and clear additional space.
   
   Like before, you need to connect to your Garmin. This time, the language files are located in \Garmin\Voices. Just in case, I would first copy the languages files you don't want to a safe location on your computer before deleting them. That should free up more than enough space for the PhantomALERT dataset. If not, there's also the Help directory to review (delete the corresponding language files) and \Garmin\Vehicle (each vehicle is 1mb) to look at. Additional tips can be found in this garmin answer.