Routing Differential Lines vs PCB Fabrication Constraints
The only parameter you can change to solve this is the substrate height. Either use a thinner PCB or if that is not enough to get sufficiently narrow lines, place a ground plane below the trace - this means you need a multi layer PCB.
Check if they will guarantee the impedance (material selection and actual stack-up). If not, this may require you to request a controlled impedance production which of course will be more expensive.