The example contains a comma splice, plus other errors. Sadly, this is not surprising.
Change the comma after life to a period. Add a comma after However (which is capitalized because it’s now the first word of a sentence). Add a comma after error (due to clerical error is a nonrestrictive phrase: you could delete it without changing the essential meaning of the sentence). Also add a period after the last word of the second sentence (for reasons that should be obvious).
Each person is assigned a guardian angel who helps them throughout their life. However, due to clerical error, a demon was assigned to you instead.
I changed everyone to each person because everyone is often used to mean an indeterminate number of people collectively. In this example, that would mean a group of people is assigned a single guardian angel, which seems unlikely, given what’s said in the second sentence.