Changing a cassette and adjustments after
If you can change to biggest sprocket on the small chainring, sounds like the chain is too short. If it's new you can add a link in (if you have the left over links), otherwise you'll need to replace and resize the chain.

You should probably also check that your rear derailleur can take a 34 tooth max cassette sprocket, and you have not exceeded the derailleur total capacity (difference in chainring tooth counts plus difference between largest and smallest sprocket tooth counts).