Error Message "emission System Service Required"

I have just installed new exaust system into my 2001 volvo v70 t5 and the catalysator was removed and in its place was put resonator and the lambda sond was installen on it. also I have put new tail pipe. Everything sort of would be ok but the computer started showing "emision system service required". And the car does not go as it used to go. What could be the problem and how could i solve it. Thanks.

It's because you removed your cat. There is a O2 sensor behind it to measure the cats efficiency. You need to get a ECU remap that will get rid of the Rear O2 input or buy a module that will fake it. It's a common issues when you remove the cat on OBD-II cars.

Thank you for your reply. I would like to ask you one more question those things that you mentioned:
Where could i find people that could do those things and what it could cost to me?
Thank you.