I am trying to count the number of unique suppliers in a column that
exist on a list on Sheet 2 - I can get it to count unique, but when I
use the criteria of the other list I get a #DIV/0! Error.

[These are both array formulas]

This counts unique records:

=SUMPRODUCT((T10:T2104<"")/COUNTIF(T10:T2104,T10:T2104&""))

This bombs:

=SUMPRODUCT((T10:T2104<"")/COUNTIF(T10:T2104,'Current Supplier'!
A2:A2287&""))