"They are planning to travel" was marked wrong and corrected to "They are planning a travel" instead of "They are planning a trip."
Yes, it was, be cause подорож is a trip (noun). The word for 'to trip' (verb) is подорожувати.
The might be planning a travel without planning to travel.
For example, they are planning a travel for someone else.
"They are planning a travel". Why wrong, explain please.
I don't think "travel" can be used as a noun like that in this case. "Travel" is seldom used as a noun, but when it is, it's uncountable, so you can't say "a travel".