cannot reset security questions, how to fix this issue?

See this Apple article: If you forgot the answers to your Apple ID security questions

If you forgot the answers to your Apple ID security questions - Apple Support

Scroll to -> Reset your security questions

1. Go to iforgot.apple.com.
2. Enter your Apple ID, then select Continue.
3. Choose the option to reset your security questions, then select Continue.
4. Enter your Apple ID password, then select Continue. 
5. Follow the onscreen steps to verify your identity. The information you’re asked to provide may vary based on your account details and other factors. If you don't get any options to verify your identity, you can't reset your security questions at this time.
6. Choose new security questions and answers and select Continue. See our tips for choosing security questions and keeping your account secure. 

After you reset your security questions, you might want to set up two-factor authentication instead. It's the best way to keep your account secure, and you won't need to remember any security questions. You can set up two-factor authentication right from your iPhone, iPad, or iPod touch with iOS 9 or later or your Mac with OS X El Capitan or later.

If you answer your security questions incorrectly too many times in a row, you temporarily can't use your security questions. Try to reset your questions after the waiting period ends.

If you have two-factor authentication enabled, there are no security questions. 

Then you can't forget and you don't have to reset.

See -> Two-factor authentication for Apple ID -> Two-factor authentication for Apple ID - Apple Support