If it is not possible to switch between operation modes, this is usually due to inconsistencies like if the number of work processes is defined differently in different places in the system. These are the instance profiles at operating system level, the instance profile in the database, and the definition of the operation modes themselves.
So you need to verify number of work process configured in system and in your operation mode. In screenshot, its is not correct, at one place it is 21 and another 31.
best regards
ashish