Can I fix the iPhone unavailable problem without erasing iphone

Unfortunately, no. The phone will need to be reset which will wipe all data from the phone.

Click on the link for instructions on how to reset your passcode without a computer if you have iOS 15.2 or higher.

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Click on the link for instructions on how to reset your passcode using a computer.

If you forgot your iPhone passcode - Apple Support

No, you can't. Even if you go to your Apple Store, the phone will need to be erased. Assuming you've been backing up regularly, you'll be able to restore your backup once you've followed the directions in the related article link at the top of this page. If you haven't backed up, your data is gone.