There is no such command. This "command to re-read the partition table failed" is because the card is 'locked' by the system, somebody has still something opened on the card. Either some partition is mounted and/or you have swap enabled or file opened in some application.

Best is to open card door (or battery cover for internal N800 slot) before doing something with sfdisk or mkfs. Once door is opened system will try to unmount the card and will not touch the card so sfdisk or mkfs can be safely used and should work. Or you will see system dialog about card being locked when opening the door. If you see it, don't continue with sfdisk but close door and try to close applications or otherwise free the card and try again

It is better to do the open door,sfdisk,mke2fs,mkdosfs,close door phase in one go without reboot (and without "The command to re-read the partition table failed" message of course) to avoid the card corrupted message from File Manager after reboot.

Too bad this cannot be used for internal N810 card.