How do i fix the TTDSI error has occurred please turn off ds and turn on again.

If you had used the DSi's WIFI to update the console's internal system, it will cause you not to be able to run any rom flash cartridge cause I heard Nintendo designed an update to block it off entirely. If there is a retailer that can downgrade it; that would fix it. But I'm not sure in your area/country there is a retailer that could do that, cause in my country we got plenty of shops providing such services cause here we use a lot of pirated stuffs openly. Or you must look around in the internet for one that match your console's product serial/region (not even sure it exist in the internet). If not, then you'll have no choice but use an original cartridge from now on.

Read this article: http://en.wikipedia.org/wiki/Nintendo_DS_storage_devices
In the paragraph on Supercard DSTwo, last sentence mentioned about what I said above about DSi update from Nintendo contain blocking program to deny use of any rom cartridges.

Or read this: http://www.dsihacker.com/dsi-firmware-hacked.htm
Telling people to avoid updating their DSi with newer console system firmware v.1.4 but someone had cracked the blockage. You might want to try looking about how it was cracked.

Cause I heard Nintendo clearly made an update downloadable for DSi, and any newer versions of their console to block off the use of rom flash cartridge such as R4, Supercard, etc. Some sort of anti-piracy for the console internal system to refuse reading the cartridge. Which is why everyone has to avoid updating their console's system.

---BUT---

If the issue is you had accidentally updated your DSi's rom cartridge card(e.g. R4i SDHC, Top Toy (TT) etc.) with a kernel version/firmware that is not meant for your cart type, then simply plug in your micro sd/SDHC to your PC and re-format it, just make sure you know also if its FAT 16 or FAT 32, then you install a kernel version including firmware that had previously work. You can get it at: http://www.linfoxdomain.com/nintendo/nds
Just do not forget to move all your game and save files first to your PC before formatting it.