SAP Note 28489 - Selection with mouse not possible in PReq lists

Symptom:

In various purchase requisition lists, a checkbox is displayed for the selection of each purchase requisition item. However, the field is not ready for input (that is, it is not possible to select it with a mouse.
The list which you receive when you search for purchase orders during the goods receipt or when creating a shipping notification is also affected.
Transactions: ME55, ME56, ME57, ME5A, ME5K, ME5R, MB01, VL31

Cause and prerequisites

This is not an error. In Releases 2.1 and 2.2, the checkboxes are actually not ready for input due to reasons of performance:
If you could select or enter fields in a list, the program would have to process all the lines in the list for every chosen function to find out which items had been selected. This can lead to excessively high response times if the list is long (and purchase requisition lists are generally fairly extensive). The runtime, however, does not depend on how many items were selected (or if items were selected at all). If you select using F9 or the select functions (see below), the loop via the list lines is no longer necessary. Instead, the items can be selected directly in the internal table.

Solution
To select an individual item, position the cursor on the line in question and press F9. (The procedure for deselecting an individual item works in exactly the same way.) To select several items, the following select functions are available in the menu bar (Edit -> Select).
In Release 3.0, it is possible to allow explicit selection via Customizing. To do this, choose Transaction OMGK and then 'Default (SAP transactions)'. For each transaction, you can determine whether selection with a mouse should be possible or not.
(N.B. This only affects purchase requisition lists, not the list of purchase orders from MB01. The explicit selection is still not possible.)