can I use Bref php serverless to install sendy?

I would recommend reaching out to Sendy and Bref to see if it's possible to make it work. However, looking at the getting started guide for Sendy (step 3), it notes that /uploads folder permission should be set as 777. However, on Lambda, the only writable directory is /tmp unless you use EFS to mount a directory. If the uploads directory path or any other directory path to write files can be modified, it may be possible (from Lambda perspective).