Recently ive realised the transferring stuff from older forza games like Motorsport 7 is broken, just giving people a heads up if you cant seem to import old stuff from the older games. Hopefully this gets fixed soon as theres some stuff Id love to have on Horizon 4 from my Motorsport 7 designs 
By broken you mean�
He means ânot working anymoreâ 
Itâs recognised as a âknown issueâ by Forza Support and came about after the update to optimise the game for the new Series X/S consoles back in early Nov. last year:
Series 29 Update bugged
Since that update, players on Xbox One X/S consoles and PC are no longer able to import designs, vinyl groups or tunes from older titles. As far as Iâm aware the function still works on the Series X & S but FM7 now crashes when people try to create anything in the vinyl editor.
Previous threads on the subject might give the OP more info:
Paint & decals not syncing?
Cannot import files from previous titles
Vinyl groups and full designs wonât sync from FH2,3 and FM5,6,7 to Forza Horizon 4
Horizon 4 - Canât see my past liveries / decals
Importing Tunes from FM7
Files not syncing from FM7
Importing designs from older games, bug?
Iâm sure there are more but you get the idea. As always, Iâd still recommend the OP submits a ticket in the dwindling hope that the more they receive might just convince the Devs to give it a higher priorityâŚ
Work Around Found! Requires GamePass
At the moment I have found on Game Pass, if you open a stream into Forza Horizon 4 on another device rather than on my Xbox One, then when I go to import my Vinyl Groups from previous titles they all appear. So I went through on Streaming and imported all the Vinyls I thought I might want and saved them in FH4. Closed the streaming and opened up on Xbox One and theyâre all there saved in FH4 to use.