What version and build are you running on each computer, e.g. 2020.1(1928)?
What syncing method are you using?
Is Dropbox client installed on the C drive of both computers?
Is it the same Dropbox account on all computers?
List the devices in your setup before trying to add your new computer?
Were you syncing these devices and if so which is the primary computer syncing set under FILE - SYNCING
What steps did you take to sync your new computer.