Your guess is as good as mine, this is not proper Hebrew. In my opinion a language cannot be הכרחית. Knowing the language can be, and you'd often see in job requirements something like -
"ידיעת עברית הכרחית"
But the course was made by natives. If this is wrong, has it been brought to their attention?
You can see the discussion Eurovision contestants Miki (Spain) and Kobi Marimi (Israel) had about essential phrases to know in Hebrew Miki asked, I need to know: how to ask for a beer, and where's the hospital? (There are a few others)... The best (hilarious) part is Kobi's response to what he actually thought was essential to know when visiting Israel: https://youtu.be/7-lRIJNVwEQ
I believe the German word "nötig" would match here. (Er lernt das nötige Hebräisch.)