The multi-user feature is only available for the Premium version and higher

Users can't customize windows unless given full rights.

Step 1: Give full rights to the user

1. Open your company file.
2. Login as sysadmin in single user mode.
3. Click on Setup.
4. Click on Set Up Users...
5. Enter your password for sysadmin.
6. Click on the username which you want to customize the transaction window with.
7. Click on Modify.
8. Give full accounting rights to the user by checking the box Full Accounting Rights.
9. Click OK.

Step 2: Customize the window

1. Open your company file again.
2. Login as the user you just granted full rights in single user mode.
3. Open the transaction window you want to customize.
4. Click on View.
5. Click on Customize transaction window.
6. Customize the transaction window as you like.
7. Click OK.

Step 3: Remove full rights

1. Open your company as sysadmin in single user mode.
2. Change the user rights by following DocLink: How to allow or restrict users rights for certain modules in Sage 50?