If you are able to to see your party in drop-down list while generating invoice,you can check these things:
Step 1:Make sure your party should be created in your company in BUSY accounitng software.
Step 2:If your party has been created,please go to the administration,click account master,and open the party's account in modify mode,check which group you have mentioned here,
Step 3:If the group has been incorrectly taken,it will need to be changed to debtor or creditor.
Step 4:After that you need take backup then to go to Housekeeping menu,
Step 5:Here you can run the process Regenerate Master Help Files,to update you account masters.
Step 6:When the process will complete,then you have to go to the voucher enter again and your party will be shown in the dropdown list.