Assuming your league deadline hasn't past and the game(s) you want to pick hasn't started, the issue you are having is very likely due to your manager having the 'Pick Grace Period' set too low, probably 0. This setting is the number of hours after you make your pick that you are allowed to change it. The default is the max setting, 255, that essentially disables it. Managers occasionally set this to 0 without understanding what it entails.
To see what your 'Pick Grace Period' is set to, go to Settings -> League Settings. You'll see the setting on the left-hand side in the BASIC SETTINGS box. Contact your league manager if you believe it is not what was intended. A 'Pick Grace Period' set to 0 is typically only used by Play Money pool managers who want behavior similar to a sports book where you can't change your mind after making picks.