You have requested a BUFFER dump, rather than a BUFFERS dump. The BUFFER dump is intended to do a level 10 dump of a particular buffer. The level number to specify in the event syntax is the decimal tablespace relative data block address. Of course, there is no buffer with an RDBA of 10. That is why your trace file was empty.

The levels for the BUFFERS dump are as follows:

level 1	dump the buffer headers only
level 2	include the cache and transaction headers from each block
level 3	include a full dump of each block
level 4	dump the working set lists and the buffer headers and the cache header for each block
level 5	include the transaction header from each block
level 6	include a full dump of each block

Most levels high than 6 are equivalent to 6, except that levels 8 and 9 are the same as 4 and 5 respectively. For level 1 to 3 the information is dumped in buffer header order. For levels higher than 3, the buffers and blocks are dumped in hash chain order.

I am trying to take a buffer dump with the following command

    alter session set events 'immediate trace name buffer level 10';