Issue with trailhead challenge "Using Formula Fields"

Hi I am currently progressing on developer trailhead and i am currently on Formulas and Validations >> Using Formula Fields. I am not able to identify what exactly i am missing here.

"Challenge Not yet complete... here's what's wrong:
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Delete failed. First exception on row 0 with id 001b000003Jz2fSAAR; first error: DELETE_FAILED, Your attempt to delete Test Account could not be completed because it is associated with the following cases.: 00001053 : []"

can someone help me on pinpoint what it might goes wrong?

Thank you!