Invoice number disapears when i print or preview / i have to put invoice number in the memo box to show in email or print

We have two things to check, Jswi. I'll guide you along. 

First is let's make sure the customer transaction numbers is turned on. This way, the invoice number will show. Here's how:

1. Click the Gear icon.
2. Select Account and Settings.
3. Go to the Sales tab.
4. Click the Pencil icon in the Sales form content row.
5. Tick the Custom transaction numbers box.
6. Click Save, then Done.

Once done, go back to the invoice, then check if you can see the number already.

If it's still not showing, the second thing we need to check is the invoice template. It's possible the Form numbers box is not checked. Here's how:

1. Click the Gear icon.
2. Go to Custom Form Styles.
3. Select the invoice template, then click Edit.
4. Go to the Content tab.
5. Click the Pencil icon in the template's first section.
6. Put a check mark in the Form numbers box.
7. Click Done.

Then, let's open the invoice again. Click Customize, then select the template. 

Once done, print the invoice again, and check if you can see the number already. 

After trying the steps, you can keep us updated. We're here to lend a hand.