It can be very difficult to schedule a meeting with someone if you can't check their calendar and free/busy information to know if they are available. There are several reasons why you might not be able to see someone's free/busy information.
The attendee isn't in your organization. By default, Outlook can't show free/busy or scheduling information for someone who isn't part of your organization. If you regularly schedule meetings with people at another organization, your IT administrator can set up calendar sharing between the two organizations.
You're creating a meeting in a calendar that isn't part of your organization. For example, if you have a Microsoft 365 email account and a Gmail account set up in Outlook, and you use the Gmail email address as the From address in the meeting request, you won't be able to see free/busy information for attendees. Change the From field to your Microsoft 365 or organizational email account.
Outlook has a limit on the number of attendees in a distribution list that it can display free/busy information for. If you try to add a distribution list that is too large, the free/busy check will fail. Expanding the distribution list might allow Outlook to recover the free/busy information for the attendees.
Outlook has a limit on the number of attendees in a meeting that it can show free/busy information for. Create a second meeting to check free/busy information for a smaller set of individuals.