Material &1 locked by assortment functions being performed by user &2

What causes this issue?

One or more of the materials you wish to use in this function are
currently blocked by other functions.
Example:
You wish to edit an assortment module, but the materials it contains
are either blocked in material master maintenance or are also contained
in another module that is currently being edited.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Repeat the function later or cancel it.

Error message extract from SAP system. Copyright SAP SE.