Hi, I own a Garmin Nuvi 2599 LMT-D.
At startup I get the following error message:
SYSTEM SOFTWARE IS MISSING.
Who can help me to make this work back
to get? Thank you

At its most innocuous, "System Software Missing" just means that the firmware (Garmin correctly calls it 'software') is corrupted or incomplete. In that case re-flashing the original firmware as an RGN via Updater.exe in preboot mode will restore it. As nuvi 25x9 series has two different HWIDs and firmwares, you must be sure to use the correct fw for the device. Use G7ToWin (search for it) to check its HWID or look in the GarminDevice.xlm file from a backup if you have one. Look for the 4 numerals as shown for example XXXX in the sample below. You'll find that info at the start of the XML file opened in a text reader or web browser:
<Model>
<PartNumber>006-BXXXX-00</PartNumber>
<SoftwareVersion>xxx</SoftwareVersion>
<Description>nuvi</Description>
</Model>

Less commonly the message might indicate a very serious problem such as wrong bootloader/ramloader has been flashed or a fatal hardware failure.

Give more information on the device history before the message appeared if you need more help.