Term Store Management Tool Access issue

You mentioned you are a "SharePoint Online Administrator", do you at least have the "SharePoint Administrator" role (defined at the Office 365 level) assigned? You need this role to be able to grant yourself the Term Store Administrator permissions. Otherwise, you should ask someone with this role to grant your account the Term Store Administrator permission. You also need to access the Term Store Administration from within the SharePoint Admin Center instead of from within your site collection