It would help if you described what you did and what error you got.
The procedure is pretty straightforward :
To dump Nintendo Switch Online (NSO) emulator application and extract the game ROMs, you will need the homebrew application NXDumpTool (https://github.com/DarkMatterCore/nxdumptool)
Description from my AI overlord :
Ensure you have the latest NXDumpTool .nro file installed on your SD card. You will also need your console's prod.keys generated by Lockpick_RCM.
Boot your Switch into OFW and launch all NSO apps you wish to dump at least once. This ensures they are fully unlocked and successfully decrypted. If you are using the emummc and were using the nro apps already, then skip this step.
If you never used the nro on the emummc, you need to go to the OFW, run the nso and make a new emummc.
Don't connect your CFW to any Nintendo servers are this will likely get your OFW banned.
Launch the CFW and open NXDumpTool via the Homebrew Launcher.
Locate the NSO Titles: In the main menu, select "User Titles" and choose the NSO app you want to rip.
Dump the RomFS: Navigate to NCA / FS dump options > Dump Update (Program #0) > FS Selection (RomFS) and select "Start NCA FS section dump".
Locate Your Files: Remove your MicroSD card, insert it into your computer, and locate the dumped files in the sd:/atmosphere/contents/[GAMEID]/romfs/titles folder.
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