I am not able to assign reviewer or I am not able to send my AE recommendation

Manuscript Central is trying to be smarter than you and MC assumes that 2 reviewers are needed. In the right side of the screen please change it to any numbers you need. If you put large number there (for example 9) MC will let you appoint new reviewers, but you will not be able to submit your AE recommendation unless you will change this number back to small number (for example 0). It is annoying but you can beat the system by changing this number up or down. Please remember  to press the save button after each change.

For example if you would like to make your recommendation without sending it for review (out of scope, or manuscript is not readable) then you have to change this number to 0 first and then MC will let you do the job.