A question is considered basic if the answer can be found easily in the docu
For an IMG activity the docu is directly available in front of the activity:
looking at your screenshot: it is a grey field, such fields have not been changeable for centuries in SAP
go into the detail. You can see the value of the purch req. is still grey, and it is not even listed as detail, it is listed above. Have you seen similar screens? e.g. in material master you can see such on top, in condition maintenance and many other transactions too.
What does that mean?
F1 is again not far away. put the cursor in the grey value field and hit F1
and you get: Together with the item category, the material group and the plant, the purchase requisition value determines which release strategy is chosen for the requisition.
This tells you that those values are actually key fields
Key fields are not changeable, keys can only be delete and new created.