Put your UEI in that field. The DUNS number stopped being the federal entity identifier when SAM.gov switched to the UEI, which SAM.gov generates itself, so a county form asking for DUNS is just an old label nobody updated. If the field rejects it, that's because it was built for digits only and the UEI has letters. Email the county's purchasing office and ask how they want it entered. Our county vendor form had the same leftover field and purchasing told us to type the UEI in and move on.
Your EIN is your tax ID. The IRS issues it, and SAM.gov checks it against IRS records when you register, so the legal name and EIN you give the state portal need to match what the IRS has on file, character for character.
The entity number isn't the UEI. SAM.gov gives you the UEI and the CAGE code and nothing else, so any other number on a state portal belongs to the state: either a vendor number the portal assigns after you register, or your business filing number from the Florida Division of Corporations. The field's help text or the portal's vendor guide says which one it wants.
Local buyers check a UEI on SAM.gov without an account. The public entity search shows the legal name, UEI, CAGE code, registration status and expiration date, so a county buyer can confirm you in a minute.