First of all you should know all these information taken from this official url https://support.google.com/business/answer/7107242?hl=en&ref_topic=4854193
You need to create or claim a Google My Business listing before you can start the verification process. You may see one or several types of verification depending on the type of business listing you manage:
Verify Business Listing By Post Card:
To verify your business listing by postcard, enter your business address in Google My Business. You’ll be sent a postcard with a verification code. Most postcards arrive within 12 days. Before you request to verify your listing by postcard, make sure that your business address complies
Desktop
To request a postcard:
To enter the verification code:
If your postcard is lost or missing, you can request a new one.
Mobile
To request a postcard:
To enter a verification code:
Once you’ve entered your verification code, your business information will be eligible to appear live on Google Maps, Search, and other Google services.
Verify Business Listing by Phone :
To verify your business listing by phone:
Desktop
Mobile
Once you've entered your code, your business information will be eligible to appear live on Google Maps, Search, and other Google properties.
Verify Business Listing by Email :
Email verification isn't available for all businesses. If you don't see this option when you attempt to verify, try another verification method.
Before pursuing verification by email, make sure that you can access the email address shown in the verification screen.
Desktop
If you're returning to verify after creating your listing, visit Google My Business, choose the business you'd like to verify, and click Verify now.
Mobile
If you're returning to verify, open the Google My Business app or visit https://business.google.com and choose the business you'd like to verify.
Once you've entered your code, your business information will be eligible to appear live across on Google Maps, Search, and other Google properties.
Bulk Verification :
If you manage 10 or more locations of the same business, your business listings may be eligible for bulk verification you can check more about this as mention above first line of this blog
Faq's Ask By user if they have any issue or want to talk support team :
Google Business Customer Service Team
Google customer service number
Customer Service Representatives for Google Faq's