Report Builder is compatible with MS office 365 ProPlus, you might need to check with your IT team as it could be possible due to some firewall setting on the organization network, Add-In is not working as expected.
I have a user with the same problem. The user was on v5.6.38 and this worked fine. I updated to v5.6.50 as the app requested, but now the add-in tab in Excel does not show when I enable the add-in via Excel Options. I had to re-install the old version so the user can carry on using Report Builder.