I see there have been previous posts on this - but they are ancient - so forgive me for starting a new thread.
I, too, encountered problems installing Avast U3 edition on my brand new U3 Sandisk. I finally got round them by downloading the portable .exe version (rather than the .u3p version U3 requests you use) and simply renaming it avastwoteva,etc.paf.exe (the paf bit was what I added). Then using portableapps to install it. Hey presto - it installed satisfactorily as a U3 program in the desired U3 part of my pen drive, rather than the portable apps part of the drive - which I normally use for portable programs which are not available as U3-friendly. It’s good to have it in the U3 part because it means it loads at startup.
I hope this helps others who may have encountered a similar problem. But it does seem that the U3 people have changed something which makes the Avast U3 editions difficult to install nowadays.