Add ISERROR to resolve DIV/0 error

Not sure how you'd get a #DIV/0! error from that formula (unless one of the referenced cells contains #DIV/0!)

In general, though, you can just use

=IF(ISERROR(your_formula),"",your_formula)

where your_formula is just the original formula without the first =

[or with your version try IFERROR, i.e.]

=IFERROR(IF(K51>=$N$28,1,IF(K51>=$N$29,2,IF(K51>=$N$30,3,IF(K51>=$N$31,4,IF(K51<=$N$32,5,""))))),"")

There may be better ways to approach this, though, what values are in N28:N32?