This might depend on your host. Try 755 and if that doesn’t work try 777. As a note, the WordPress developers intend for the wp-content
folder to be writeable.
Before you leave ANY folder writable you must be aware of the security implications of that.
777 is a very bad idea.
The lowest possible number is best, but if it’s 777 only that works that is not good.
You might also want to bear in mind that the caching doesn’t help a lot of sites so if that is the only reason to have that directory writable, why not switch caching off?