=IF(COUNT(K13:L13)<2,"",NETWORKDAYS(K13,L13)-1)

This formula already excludes weekends from the calculation.

Do you know how I could improve this formula so that it does not include
holidays in the calculation, I can specify the holidays, and I will not get
an error message if Column K is blank?

Or

Do you know how I can improve this formula so that if Column K is blank I
will not get a #VALUE! message where the calculation should be?

=NETWORKDAYS(K13,L14,Holidays!A1:A29)-1