How do you export a database schema from SQL Server Management Studio to Excel?
You can link your server as Data Source in Excel and then run this query:

SELECT * FROM information_schema.tables WHERE TABLE_TYPE='BASE TABLE'


This will export data into Excel

How to connect SQL server to Excel: 
https://support.office.com/en-us/article/Connect-a-SQL-Server-database-to-your-workbook-22c39d8d-5b60-4d7e-9d4b-ce6680d43bad

Or if database is not that big then just run it in Management Studio and copy-paste the result to Excel