Calling subtemplate gets me an error

in may main template, I have the following code:

<?import:xdoxsl:///Custom/Procurement/POTemplate/filename_of_subtemplate.xsb?>

<?choose:?>

<?when:?><?xdofx:substr(Comments,1,3) = 'ORD')?>

<?call-template:P3PO?>

<?end when?>

<?when:?><?xdofx:substr(Segment1,1,3) = 'CON')?>

<?call-template:OptimusPO?>

<?end when?>

<?when:?> <?xdofx:substr(Segment1,1,3) = 'PUR')?>

<?call-template:CustomPO?>

<?end when?>

<?end choose?>

I have tested each templates and they are working as expected.

I am getting the error below when I tried using it as subtemplate