The manifestation process is always happening whether we are aware of it or not or in other words we are creating our reality, only sometimes we are not doing it consciously or deliberately.

Karma is a philosophy that implies that our cuurrent situation or future is already predetermined by our past actions. I personally believe that there is often a lesson within those past actions that we may not have got and until we get that lesson or in other words pay that karmic debt, to a degree we really don't move forward in our journey because the lesson will only keep presenting itself in another guise until we do get it.

I think you may be asking do we still have the power to choose our manifestations if we believe in karma? It may hinder our manifestation process if the thing we want to manifest is in some way detrimental to the lesson we need to learn. However, everything that manifests in our physical reality happens because of our choices and is always a mirror to how closely we are aligned with our Source.We always have the power to change our destiny by realizing our mistakes and forgiving ourselves and others along the way which is really just another way of settling that karmic debt.

If our life situation reflects misery and suffering,we can choose to change it by endeavoring to get the lesson we previously missed by introspection and working on our own self growth and aligning our thoughts with Source or God and as a result the manifestations we create begin to change because our physical reality is always a reflection of our inner state.

Karma and 'manifestation' are both related through attachment-to and beliefs-about "reality". There is no such thing as 'good' or 'bad' karma. ALL karma leads to suffering and all 'manifestation' generates karma.