It is sometimes necessary to create a temporary join in ModelBuilder, and later remove the join from the model. However, when using the Join Field tool to join the field, and using the Remove Join tool to remove the join, the process fails, and the following error is returned:
ERROR: 000229: Cannot open "file name" Failed to execute (Remove Join)
This is because the Join Field tool creates a permanent join, and therefore the Remove Join action does not remove the join and errors out.
The Remove Join can only remove joins that were created "in memory", such as by the Add Join tool.