After migrating, I am getting this error for several of my buttons and menus on Craft 2.6 Internal Server Error
Craft\MatrixBlockModel and its behaviors do not have a method or closure named "header".

Apache Version  2.4.37
PHP Version 7.1.24
MySQL Version   5.7.24
Architecture    x86_64
Operating System    Linux Cent OS
This error indicates that you don't have a matrix block, called header, setup in the CMS, but your template is expecting one.

The solution is to create a new matrix field;

settings > fields > new field.

The field type should be “matrix” and it's handle needs to be “header”.