There are several common reasons why you may have trouble connecting to the SFTP server.
You haven't created an SFTP account
Before you can upload files via SFTP, you need to create an SFTP account. Note, your SFTP and Google Merchant Center account logins will be different. Therefore, they'll have different usernames and passwords.
Your SFTP account hasn't become active
You may be unable to log in when attempting to connect too soon after creating or editing your SFTP account. It takes a couple of hours for new SFTP account information to become active.
You aren't connecting to the correct SFTP server
For SFTP, if you’re using command line prompt, you should be connecting to {your SFTP username}@partnerupload.google.com with a specified server port of 19321 (e.g., sftp -P 19321 {your sftp username}@partnerupload.google.com). If you're using an SFTP client, you may need to prefix the host name address with the ‘sftp://’ protocol (e.g., sftp://partnerupload.google.com).
You're using an incorrect SFTP username
You can verify you're using the correct username by signing in to your Google Merchant Center account. Once you're signed in, select SFTP / GCS under the 3-dot icon menu. If you’ve set them up, you'll see your username displayed under the Username section for each service.
You're using an incorrect SFTP password
SFTP password isn't displayed in your account for security purposes. If you've lost your password, you'll need to reset it. Learn how to reset your SFTP password.