It works for me when I do the following:
Go to Settings/Options . Select the Wallet tab. Tick the check box: "Enable coin control features". It will appear a button with the label "Inputs...". Press on that button and select the inputs you want, or if you prefer, select all inputs.
At that moment, the radio button "total at least" gets enabled and it is ready to use.
i was referring to the custom menu where you set "per kb" or not" after you set per kb, you can not uncheck it anymore
I have tried again and I can uncheck the "per kilobyte" radio button only if I select at least one input as I specified in my previous post. If I select at least one input, I can switch from the "per kilobyte" radio button to the "total at least" radio button and viceversa without problem.
I agree that it is not working 100% OK because the UI lets you have the "total at least" radio button selected and active and the "Send as zero-fee transaction if possible" checkbox ticked and active simultaneously, which doesn't make sense, but that is a different issue.