I bought a $300 Apple gift card and is activated and i found isn’t working no money on it

Please follow the steps in this Apple Support article. 

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

If you receive a request for payment using Apple Cash, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

You may also report the scam to the FTC Complaint Assistant.

In addition, never post pictures of your Gift Card on a public website such as this. Anyone can claim those pictures are theirs and possibly send them to Apple for a resolution. Nothing in those pictures says the card is yours and anyone can claim it with the information you have provided. Apple may ask you for documentations, but you would only provide it at that time using the method they tell you. I have asked for the pictures to be redacted for your protection.