If you see that a keyword or multiple keywords have 0 search volume, it could mean the following:
The monthly search volume for the keyword is low (less than 10 searches per month).
My Telescope was unable to retrieve the search volume data for a keyword. Save the search and return after 12 hours.
Google has restricted the keyword, "0" is shown for keywords that are related to prohibited and restricted topics. Google does not disclose the search volume for keywords or phrases which contain restricted keywords. For example: cannabis, vaping, weapons, pornography, etc. Read through Google Ads policies to find the complete list of prohibited and restricted content.