Which version of LT do you have?
Howard Walker
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Select all of your objects in the file (ensure all layers are unlocked/thawed/on), open PROPERTIES command, at the very top use the pulldown to sort by object type, then scroll down the long list to ever "Z' entry and manually replace whatever is written there to 0 (that's a zero, do it even if you see a zero).
Read all about it (and more) here from Autodesk
If it takes you more than a minute for a DWG file I'd be very surprised.
FLATTEN command in full AutoCAD also destroyed 3D Solids and 3D blocks to make then 2D: there is literally nothing in LT, even with LISP in 2024/2025, those core commands are just missing.