The sample XML:
<root>
<repeatsection>
<repeatline>
<fieldx>content</fieldx>
</repeatline>
<repeatline>
<fieldx>content</fieldx>
</repeatline>
</repeatsection>
</root>
Then the design, a subform for the section and one for the lines repeating and a field

Set the repeat to min 1 - no other setting required

and your field

Not sure where you see this warning - is this a triangle in Designer? Try this approach and tell me if you still get that particular message...