To resolve this, you need to convert your private key to the latest format using the Sui CLI tool. Download the CLI and use the command 'sui keytool convert'. Documentation can be found here: https://docs.sui.io/references/cli/keytool

If your old private key is truly invalid, it means it's incorrect or corrupted, and you won't be able to import your wallet using it. The private key is the credential that grants access. Your only option would be to use your mnemonic seed phrase (the 12 or 24 words) to recover or import your wallet. If you don't have a valid private key or your seed phrase, access to that specific wallet address is unfortunately lost.

If the error message you see suggests that the private key is invalid it also probably advises using a Bech32 encoded 33-byte string. Check details how to solve it here: https://github.com/sui-foundation/sips/blob/main/sips/sip-15.md