How journal entries calculate billable expenses account as item account type debit or credit?
You should not use Journal entries for this. Look at the guides such as Record billable expenses | Manager
But When I create new journal entry in items I can choose Billable expenses as item account and then choose customer and invoice when I back to summary I show transaction in billable expenses account that has type journal entry as credit and debit as same time how come?!
No, you cannot, because journal entries do not include the Item field the way, for example, a sales invoice does. In the Account field you can choose the Billable expenses account. This is for the very rare case where you might need to make an adjustment.
As to whether the entry is a debit or credit, that is entirely up to you. Manager does not make that determination for journal entries, which is why the program is designed so the need for journal entries is rare.
Without seeing the Edit screen of your journal entry, it is impossible to know. Most likely, you entered the same value as both debit and credit.