Minimum and maximum are set on the Reorder Points screen — not on the item itself.
Set the numbers
In the top menu, click Purchases, then Item Reorder Points.
Find your item — use the Search SKU code/name box.
Type a number in the Min column.
Type a number in the Max column.
Click Save on the Unsaved changes bar that appears at the bottom.
There are two buttons called Save
There are two buttons called Save. The one in the toolbar at the top belongs to saved views and stays greyed out — it is not for your edits.
Your changes save from the floating Unsaved changes bar that appears at the bottom of the screen once you type something. If saving seems broken, you are almost certainly clicking the wrong Save.
Reorder points are per warehouse. The same item can hold 500 in your main warehouse and none in a second one. Check the Warehouse column before typing.
How to know it worked
Reload the page — your numbers are still there. Then open the reorder report: if the item is below its minimum, it now appears.