Access is layered. Someone can be in your workspace and still not see a sheet, because being a member of the workspace is only the first of four gates. Check them in order — the answer is almost always the first one that fails.
Open Settings → Users. If their row says Invited, they never accepted — resend the invite. An invite that was never accepted looks exactly like a permissions problem.

Sheets live inside spaces, and a space grants access to what it contains. If they are not a member of that space, nothing inside it is visible to them. This is the most common cause.
A role sets what someone can do once they have access — read, edit, or manage. A guest-level role may be able to open a sheet but not change it.
A sheet, and an individual view, can be narrowed to specific people. If everyone else can see it and one person cannot, check this last.
Related: Invite your team · Views: see the same data 12 different ways