Wheel conversion to Fixed gear (threaded hub)

Fixed cog can't be properly installed on your wheel. Hubs intended for use with track cogs have a separate smaller reverse thread for the lockring. The reason for reverse thread is that when you apply reverse torque, it unscrews the cog but tightens the lock ring so that the cog stays in place.

If you absolutely have to use track cog with this hub, there are bottom bracket lockrings that fit the same thread. With thread locking compound and lots of torque it sort of stays in place. This setup is called suicide hub, and it really should not be used except with rear brake and very careful braking.