What do I need to do if a credit card company is sending me letter about cc inactivity?

Contact the credit card company and request them to not close the credit card. Use the credit card frequently and responsibly.

Once a creditor closes your card, the first thing you should do is to get a copy of your credit report and check whether the account is properly reported as “closed” or “closed at lender’s request.”
After that, ask the creditor for an explanation of why the card was canceled. Also make sure to reassess how you’re currently using credit.

Ask the reason.

It is better if you can visit the issuer physically or contact them over phone, and explain the reason for inactivity and do the needful as suggested by them.

As per the CARD Act of 2009, creditors must inform customers by giving a 45 days notice regarding any major changes to the terms of their accounts. But unfortunately, a card cancellation caused by inactivity doesn’t belong to such major changes.

All you can do is to get in touch with your card issuer right away and negotiate to have the card reopened.