How can fix the Error using dir Function is not defined for 'cell' inputs?

rootpath = {};

D = dir(rootpath);

[Stephen23]

Answer: Don't use a cell array for the input.

Read the dir documentation. It clearly states that the input must be a string.

Answer 1

[Guillaume]

The fix is simple, don't pass a cell (an empty one at that) to dir.

It's not clear what you're trying to do. rootpath must be a string that points to a valid path on your machine. Maybe,

rootpath = 'C:\';

but only you know what it should really be.