I think 'Target' should be 'TARGET'.

Since you specified 'OSD=messui' i don't know why it says 'winui' is unsupported, since you didn't say 'winui'. Unfortunately, the rest of the error message is meaningless.

Also, I haven't tested if TOOLS=1 works, maybe you should leave it out and see if it makes any difference.

If it still doesn't work, I'd suggest to continue this on the official messui forum, as discussion of it is frowned upon here.