You may receive the following error when attempting to sell your tanks: Unable to sell 'X'. Please make sure that the vehicle is not in any tactical unit or try again later
This error is caused by overfull Barracks. In order to correct this error, you will need to remove any excess crew until the number of crew in your Barracks is equal to or less than the number of available bunks.
Now that the number of crew has been reduced to equal the total number of bunks, the MKA we failed to sell earlier can be sold.
If these steps do not allow you to sell your tanks, please submit a ticket to Player Support.