When I searched it on google nothing came up about this crisis.
You might be the first one who has experienced this.
And I know how to fix it.
1))Go to nintendo Eshop and press Undertale
2))Press "repair software"
3))It says "Do you want to check if this software needs to be repaired
4))Press ok
5))It should say this may take some time please wait etc,:
6))If it says "check complete 'No problems have been detected.If you are experiencing problems with this software,you can initiate the repair software anyway.Doing so will redownload the softwares data.Initiate the repair software?
Save data will not be deleted.
If you experience problems after the repair process is complete,please contact your local customer support centre.For contact details,check the operation manual or visit (rule)
7))press ok
8))then it will show download 8 blocks
9))Press download
10))It will repair the software
If these 10 steps don't work I'm sorry and I don't know what else to do.