Can anyone help me with this: I can not enter my county name under New York State School District/County Selection? it keeps saying fix your tax return! what should I do

Assuming that you entered Nassau on the Tell Us Your New York County page, the Tell Us Your New York School District screen should generate all school districts in Nassau County:

Nassau isn't an option.  You might check the New York State Education Department's NASSAU COUNTY DATA page to see Nassau County districts.