You have a GO after your USE statement. This terminates the batch above the GO, eliminating the variable declaration in the batch started immediately after the GO. Move the USE and GO above your declarations and all should be well.

USE msdb

GO

DECLARE @v_body varchar(MAX)

DECLARE @v_business_name varchar(75) = 'Hollies Gift Shop'

DECLARE @v_owner_first_name varchar(30) = 'Bernice'

DECLARE @v_owner_last_name varchar(30) = 'Goodly'

DECLARE @v_profile varchar(30) = 'testprofile'

set @v_body = 'Business Name: ' + @v_business_name + char(13) +char(10)

+ 'First Name : ' + @v_owner_first_name + char(13) +char(10)

+ 'Last Name : ' + @v_owner_first_name

EXEC sp_send_dbmail

@profile_name = 'testprofile',

@recipients = 'example@yahoo.com',

@subject = 'Test message',

@body = @v_body;

Another way is to EXECUTE msdb.dbo.sp_send_dbmail instead. I personally prefer this approach, but it's just a matter of preference.