The general pattern has been that Fusion licences include downgrade rights for one preceding major version, e.g. if you buy a version 13 licence you could downgrade it to a version 12 licence.

[edited the following to correct initial misinformation]

Per the note earlier in this thread from @Mikero and other threads here, late updates of Fusion 11 (probably 11.5.7) will accept a Fusion 12 licence key, and that version will run on macOS Mojave 10.14 so you should be OK.

Also worth noting that a free personal use licence won't work because that option didn't exist for Fusion 11. You need a paid licence for Fusion 13, downgraded to Fusion 12.

Also worth noting that Fusion 11.5.3 or earlier doesn't support Fusion 12 licence keys, so if you need to run Fusion on macOS High Sierra or earlier it is already too late to buy a new licence. (Fusion 11.5.5 through 11.5.7 require Mojave or Catalina.) When Fusion 14 is released at some point in the future, that will remove the Fusion 12 downgrade option, which will prevent getting a new licence which can run on Mojave (11.5.7), Catalina (11.5.7 thru 12.1.2) or Big Sur (12.x).