This question was submitted on 09th October 2020 to the Google My Business Help Community Forum.
You can only choose one of the verification options available for your listing.
If the only option is a postcard, you need to ask for one.
The verification methods available to each listing will depend on how much trust does Google has on your phone number, website, business existence.
Why does Google do this? To control spam/fake listings creation.
After 14 days of the postcard request and only after (if you try before Google will ask you to wait), you can use this online interactive webform to verify your listing. You will need to supply all of the info requested.
How to navigate the online interactive form:
Step 1 Mention the issue as “verification” »
» in step 2 confirm the reason, bypass the “Recommended” section and click on “Next step” »
» in step 3 click “Email” and fill the form. You will need to supply all of the info requested.