Teams button for Outlook calendar is greyed out

If you read more into my post, you'll see that at first there was no button, but then I added it to the ribbon, and that's when I noticed it was grayed out. It looks like mine (below), except without color and nothing happens when she clicks on it.

Can you explain Modern Authorization to me? That's a term I've never heard used. She's using the desktop app, not the web app. She is logged in, and she has been able to attend meetings or do video chats through it without issue. I've already gone through the link you provided before posting here, and sadly that was no help.

Again, if you read through to the end of my message, you would see that this feature is working on other PCs, and they've been able to schedule Teams meetings through their own Outlook software.

Your first statement is inconsistent, because she did have Office 365 installed on her desktop prior to installing Teams, and the add-in didn't show up. Additionally, it doesn't make sense to give someone the manual option to add it if it wouldn't work anyway. Why allow that function if it doesn't do anything?

The problematic user has the same laptop as I do, and my button works. I checked the link you provided and confirmed she's using modern authentication: "If users have already signed in to other Office apps through their Office 365 Enterprise account, when they start Teams they're taken straight to the app. There's no need for them to enter their credentials." She's already signed into her other Office 365 E3 account.

I'll download the Support and Recovery Assistant today, to see if that can tell us anything.

The SaRa was unable to help with my request. I went through the Outlook section and didn't see any options that pertained to what I need, and the Office section was only about installations or activations. I have a log file from the experience if that would help any.

"When you say “The problematic user has the same laptop as I do”, do you mean that problematic user and you using the same Outlook desktop client version?" This was in response to asking to try the problematic user on a different PC, which I now see I misunderstood. She has a slightly newer version of Outlook than I do, which doesn't make sense that when I try the Update button, it says I'm up to date.

I'll try the uninstall/reinstall of Office at her earliest convenience.

The uninstall/reinstall didn't work. I made sure upon opening that I opened Teams first, then Outlook. I even went through an Office update, uninstalled/reinstalled a second time Teams, and that didn't work either. When I looked under File > Options > Add-ins, the Teams add-in wasn't listed there, not even under Disabled ones. I'm unable to create a new Windows account for her to use as we aren't on the domain in our building due to Corona restrictions.

Her version of Outlook now matches my own.