AutoCAD LT 2015 looks for .NET 4.5. If you have .NET 4.8 or 5.0 installed, the installer might mistakenly think the requirement isn't met. You may need to temporarily edit the Windows Registry (HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full) to trick the installer into seeing version 4.5, though this should be done with extreme caution.
If online activation fails, look for the "Request an activation code using an offline method" option. You will need your Serial Number and Product Key (for AutoCAD LT 2015, the Product Key is typically 057G1). Step 4: Graphics and Performance Fixes autocad lt 2015 download fix
Even after a successful installation, you might face an "Internal Error" during activation. This is often caused by the retirement of the older activation servers. AutoCAD LT 2015 looks for