If you face a situation where you need to plugin a database that has a different block size than the destination CDB then you need perform the following:

Let us assume that the source database that will be plugged is configured with 16k block size , but the destination CDB environment is configured for 8k as shown below:

SQL> show parameter db_block_size

In the CDB$root container execute the following:

SQL> alter system set db_16k_cache_size=200M scope=spfile;

//note i am using db_nk_cache_size parameter

Restart your CDB instance. And then try to plug-in the database.

Always check pdb_plug_in_violations view for any issues:

SQL> select message,time from pdb_plug_in_violations;

If I try to create a tablespace with 16K size ….. you will be able to create a tablespace successfully  with no issues: