"Could not lock data base:read only file system"

I did a fresh istall on my dell laptop (Vostro 5459) with automatic partition.When it finished,I tried to install some apps with terminal (console) (sudo pacman -Sy ...),and I got this error:"failled to init transaction" and "Could not lock database:read only file system".Can I do something else?(note that I have previously installed debian with no issues)