Javascript Not Staying in Acrobat Pro Form Calculation

I'm trying to add Javascript to an adobe form in Adobe Pro (this script: https://community.adobe.com/t5/acrobat-discussions/calculated-date-field-14-days-from-date-entered-in-another-field/td-p/11874307 ). But when I enter the form in Calculate - Custom Calculation Script and then select "OK", the script doesn't stay. It does the same in Validate - Run custom validation script. 

I've also tried various other scripts. Video attached. idk what's up with the quality.