Btw : I think that you never filled in what so ever, but our 'intelligent' browser will auto-fill some entries - check always everything before you validate a page.
I didn't try to mess with cookies and stuff like that - just changed from my loved Firefox to Microsoft Edge… and it worked.
There should be something wrong.
So I decided to show all my steps just in case.
STEP 1 - CREATE CAPTIVE PORTAL ZONE >>> X - Enable Captive Portal
INTERFACES - OPT1
AUTHENTICATION METHOD - X LOCAL USER MANAGER/VOUCHERS
Click SAVE - OK
STEP 2 - ACTIVATE VOUCHERS Click over EDIT CREATED CAPTIVE PORTAL
Click over VOUCHERS
Enable X - Enable the creation, generation and activation of rolls with vouchers
Click SAVE - OK
STEP 3 - CREATE AND GENERATE A VOUCHER ROLL Click over VOUCHER ROLLS
Click over ADD
Roll # - 0 (zero)
Minutes per ticket - 10 (minutes only)
Count - 5 (vouchers code)
Click SAVE - OK - Voucher created - a new roll line appear at screen
Click GENERATE - a button besides - asked for where to save a .csv file with vouchers codes - OK
STEP 4 - CUSTOMIZE CAPTIVE PORTAL PAGE Click over EDIT CREATED CAPTIVE PORTAL
Created a custom file VOUCHER.HTML to only ask voucher code
ENTER YOU VOUCHER
<form method="post" action="$PORTAL_ACTION$">
<======= DELETED THIS LINE FROM SAMPLE
<======= DELETED THIS LINE FROM SAMPLE
</form>
===============
Look down for HTML Page Contents / Portal page contents
Click over SEARCH and load voucher.html
Click SAVE - OK
5 - TEST It worked fine
Thanks to all the community again !!!