We need more info from you.
You can sync one catalog from LR Desktop to multiple mobile devices each using LR Mobile.
iMac and Mac Pro are both desktop computers (not mobile devices) so they cannot sync with each other.
The easiest way (not prettiest way) to keep both desktops synced is by putting the catalog and images on an external drive and moving the drive from computer to computer as needed.
You can also use Dropbox/OneDrive/GoogleDrive to copy the catalog from computer to computer if you are very careful.
If you are just looking to transfer everything from the old computer to the new computer, see this link:
thanks for both of your replies, both very helpful.
I realised after I posted the question that the laptop couldn't be sync with the IMac because as Maniacloe explained, it is not a mobile device. I hope Adobe look into this and in future updates, make it possible to sync all your devices.
for now the solution that I will be adopting is sending the catalogue to creative cloud and downloading it from there onto my laptop.
I'd suggest you try Mylio for synching between Macs (and mobile devices). It works very well with Lr, and can pass metadata and most adjustments back and forth. Free for JPEGS up to a certain number of files and devices.