Your old license code may have been entered into config.php. This overrides codes entered through the web interface.

Check config.php (in the directory where you installed Deskpro - you will need access to the server file system to do this).

If there's a section like this at the end:

$DP_LIC_STR = <<<DP_LIC_STR

old code

DP_LIC_STR;

define('DP_LIC_STR', $DP_LIC_STR);

delete it or enter the new code there.