Before worrying about setting your environment variables, e.g. _NT_SYMBOL_PATH, get the Windbg session working.

Try:

lkd> .symfix

This will set your symbol search path to the Microsoft msdl symbol server. Next, reload your symbols:

lkd> .reload /f

This will force the debugger to immediately reload all symbols. This should fix the issue. If it does not, try

lkd> !sym noisy

This will turn on your symbol prompts. Try again running .symfix, looking for clues in the output.

Once your symbols are fixed, you can fix your environment variables. First, ensure that you have created a symbols cache, e.g. C:\Symbols. Add it to .symfix via the command:

lkd> .symfix +C:\Symbols

Ensure that symbols still work (do a .reload), then have a look at your .sympath:

lkd> .sympath

Symbol search path is: symsrv*symsrv.dll*C:\Symbols*https://msdl.microsoft.com/download/symbols
Expanded Symbol search path is: symsrv*symsrv.dll*c:\windowssymbols*https://msdl.microsoft.com/download/symbols
************* Symbol Path validation summary **************
Response Time (ms) Location
Deferred symsrv*symsrv.dll*C:\WindowsSymbols*https://msdl.microsoft.com/download/symbols

Now set your _NT_SYMBOL_PATH using the output above.

1. Right click on My Computer
2. Properties
3. Advanced
4. Environment Variables
5. New (User variables)
6. Variable name = _NT_SYMBOL_PATH

7. Variable value:

   symsrv*symsrv.dll*C:\WindowsSymbols*https://msdl.microsoft.com/download/symbols

Close Windbg and open a new session. Ensure that symbols are fixed. If not, examine your  .sympath and ensure it does not differ from the settings that worked in your previous session.