Installing Windows 7 in Boot Camp is Unavailable on Mojave
What I would suggest is getting old bootcamp files and going the manual route, that is, without using Bootcamp assistant. You would create an exFat partition, get a bootable Win7 installer disk and boot into the disk to install Win7 on the exFat partition. Booting into Windows 7, you can then manually install the Bootcamp drivers.  
The latest Bootcamp drivers, that is Bootcamp 6 don't support windows 7/8 installation as far as I know, so you'll have to get older drivers. Here's the link for Bootcamp 5.1.5722: https://support.apple.com/kb/DL1836?locale=en_US
According to them:


  • The download file is a .zip file. Double click it to uncompress it,  if it is not automatically uncompressed.
  • Double-click the Boot Camp5 folder.
  • Copy the entire contents of the .zip file to the root level of a USB flash drive or hard drive that is formatted with the FAT file system
  • When running Windows, locate the Boot Camp folder on the USB media you created in Step 3 and double click to open it.
  • Double click on setup to start installing the Boot Camp Support Software.
  • When prompted to allow changes, click on Yes and follow the onscreen instructions.
  • Installation can take a few minutes. Don't interrupt the installation process. When installation is complete, click Finish in the dialog that appears.
  • A system restart dialog box appears.  Click Yes to complete the installation.