YOu can also rewrite it like this
FROM Resource r WHERE r.ResourceNo IN
(
SELECT m.ResourceNo FROM JobMember m
JOIN Job j ON j.JobNo = m.JobNo
WHERE j.ProjectManagerNo = @UserResourceNo
OR
j.AlternateProjectManagerNo = @UserResourceNo
Union All
SELECT m.ResourceNo FROM JobMember m
JOIN JobTask t ON t.JobTaskNo = m.JobTaskNo
WHERE t.TaskManagerNo = @UserResourceNo
OR
t.AlternateTaskManagerNo = @UserResourceNo
)
Also a return table is expected in your RETURN statement