Ah, this likely means that the field is not on a view visible to the user but
is still part of the form's data source. Go into the Design Mode and then
view the Data Source in the taskpane. Any nodes there that have a red * are
required by the schema. You may need to expand the tree to see all the
required nodes. To ensure data integrity, InfoPath will not allow submitting
a form if required nodes are empty or there are any fields/nodes with invalid
data.

You have two options. You can make sure that all required nodes are
somewhere on the form for the user to fill in. Alternatively, you can
double-click the node in the tree and set a Default Value for that node that
will always be put in by default. That way, even if the node is not shown as
a field on the form it will still have a value to be valid and allow submit.