trying to use phpSpreadsheet

"Require" and "Use" shouldn't be used inside a function, but before the class definition, at the top of the controller.

That is true of "use" but not of "require".

Not germane here, but important to know that (per the PHP manual) "A file containing a namespace must declare the namespace at the top of the file before any other code - with one exception: the declare keyword."

Right, that's why in this case, the require statement must be used at the top of the controller. The namespace can't be used if the third party php file wasn't included with require or require_once.