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 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