Unable to type letter “n” on my MacBook Pro's keyboard

Is this by any chance a 2016-2019 model? If so and using compressed air to clean the Keyboard as suggested by @varjak paw doesn't help, then you have a hardware issue with the built-in Keyboard which is extremely common with the 2016-2019 models (Apple had a free repair program for them that is now expired).

When using the compressed air, make sure to keep the can of compressed air completely upright or you can damage the laptop with the cold wet propellant. Here is an Apple article with details for cleaning the Keyboard:

How to clean the keyboard of your MacBook or MacBook Pro - Apple Support

You can use an external keyboard (wired or wireless) to get around the problem.

I personally don't recommend spending any money on repairs for any 2016-2020 Intel Macs for multiple reasons. The money spent on repairs for those models is better put towards a new laptop.

You can try blowing out the keyboard with a can of compressed air. If there's some debris under the key, that might fix the problem. If not, then you will probably need to take your Mac to an Apple Store or authorized service center for evaluation.

Regards.