Are you on Steam? Because that might be your problem right there.
You aren’t supposed to be able to import stuff between the Xbox/Windows Store versions and the Steam versions. “Due to the differences in save file structure between Xbox and Steam platforms for previously released Forza titles”, see here.
In theory, you should be able to import from Steam FH5 to Steam FM23. Although it appears that import on the Steam version just generally doesn’t work. See this known issue.