I am using Office 2016 and Office 2019 on different computers. I continually get prompted for credentials after I open outlook. How can I stop this ?

What credentials are you being prompted for? A password to open Outlook, Exchange Server login credentials, or email account password? Need a bit more info about the environment you're operating in. Take a snapshot of the prompt you're getting and upload that in your next comment if possible.

Thanks.

Do you have an option to tick, check the save password box?

Double check how many email accounts you have within to make sure which of the email accounts is triggering the credential prompt.

Where is managed your mailbox ? Exchange, Exchange Online, Other?

By default, the password has to be saved with the box (near the password zone), but it does not work in all configurations.

If the session is opened with the account corresponding to the mailbox (same login/password), the password should not be requested. In the same way, If the mailbox is on Exchange Online, the modern authentication must be enabled at organizational level, and the password will be not be asked.

I am researching this for my CTO. It's not happening on my machine so a snapshot will take some time to get. Re:What credentials are you being prompted for? I assume it's a password to open Outlook

Re:
Do you have an option to tick, check the save password box? Yes, it's ticked
                                   
                                    Double check how many email accounts you have within to make sure which of the email accounts is triggering the credential prompt.
It's using one account on OWA browser connection

Re:
Where is managed your mailbox ? Exchange Server

By default, the password has to be saved with the box (near the password zone), but it does not work in all configurations.   Not sure what you mean by password zone

If the session is opened with the account corresponding to the mailbox (same login/password), the password should not be requested. In the same way, If the mailbox is on Exchange Online, the modern authentication must be enabled at organizational level, and the password will be not be asked.
Not sure what modern authentication means. I'm fairly sure this is a flaw in outlook. I have seen it on one of my laptops with Office 2013

Re:What credentials are you being prompted for? I assume it's a password to open Outlook 

You'll need to confirm exactly what credentials are being asked for. There are too many different possibilities for anyone to be able to offer any meaningful advice without knowing for sure. For instance, the problem could be on the Exchange server and nothing to do with the locally installed copy of Outlook.

If your environment uses two factor authentication, that might explain things.

If possible, setting up an APP password and saving it, should prevent the need to revalidate username/password mfa Token.

We know what is being used, we do not know what is being accessed and who controls that.
The CTO should actually be using MFA to secure her account.

Office365?

Hello, you may have a look into the Credential Manager (Control Panel) if there are left over logon credentials either for Outlook or Sype, Lync etc.
Usually they are not needed.
If the connection to exchange is lost (i.e. reboot), the user gets a logon promt. If the user then selects "save password", the crendentials are stored there and after a password change, they may pop up as no longer valid.
You may also inspect the BadPasswordCount attribute in AD to validate such an issue.