You’re missing } before elseif:
<?php if (is_page ('20')){?>
print this
<?php } elseif (is_page ('50')){?>
then print this
<?php } else { ?>
print this
<?php } ?>
Remove the PHP tags and you’ll see why:
if ( is_page( '20' ) ) {
elseif ( is_page( '50' ) ) {
} else {
}