Thinkpads are extremely well documented. For how to repair/replace parts, you need the HMM. Just Google for “Thinkpad t14 Gen 1 HMM” and you should find the official PDF on their site. That will tell you, step by step, how to replace the keyboard.
As for the part itself, you can again check Lenovo’s site for all compatible parts (FRUs) and find the item number and details. While I wouldn’t recommend buying directly from them due to cost, this should give you the information needed to find it elsewhere. eBay has tons of Thinkpads being sold for parts, and many of these will be parted out. You should have no issues finding what you’re looking for.
If you skipped the area code, it probably failed the general validation check. To really test this, you would’ve needed to try a different (but completely valid) number