The reason you are seeing the INVALID_CREDENTIALS error when looking up a VAT registration number is because the access token that your installation of Easy MTD requires to communicate with HMRC is invalid or has expired. To resolve this, please select any business on the Businesses page in Easy MTD then click the Go button.
If VAT information such as open and fulfilled obligations are successfully shown in Easy MTD, try looking up the VAT registration number again.
If the VAT information is not shown or a Connection Expired window appears, you will need to connect Easy MTD with HMRC so a new access token is created. Start this process by clicking the Connect link for a business on the Businesses page or the Yes button if you see a Connection Expired window. Now work your way through the connection process. When complete, you should be able to successfully lookup the VAT registration number.
More: For further details about connecting Easy MTD with HMRC, refer to the Connecting with HMRC section of the help.