Check Free Space in SQL Server (License Limit)
You can utilize the below to see how much free space is left in the data file(s) as well as how much you've already used. The limit for data files in Express edition is 10 GBs currently(https://docs.microsoft.com/en-us/sql/sql-server/editions-and-components-of-sql-server-2017?view=sql-server-ver15)
The code is based off of: https://am2.co/2016/04/shrink-database-4-easy-steps/
USE DemoDB
GO

SELECT
DB_NAME(),
dbf.name AS LogicalName,
dbf.type_desc AS FileType,
fg.name AS FileGroupName,
dbf.physical_name AS PhysicalFileLocation,
CAST(  (dbf.size / 128.0)  / 1024.0 AS DEC(10,2)) AS FileSizeGB,
CAST(  (FILEPROPERTY(dbf.name, 'SPACEUSED')  / 128.0)  /1024 AS DEC(10,2))  AS UsedSpaceGB,
CAST(   ((dbf.size / 128.0)  / 1024.0) - ((FILEPROPERTY(dbf.name, 'SPACEUSED')  / 128.0)  /1024) AS DEC(10,2)) AS FreeSpaceGB 

FROM
sys.database_files dbf
LEFT JOIN sys.filegroups fg
ON dbf.data_space_id = fg.data_space_id


ORDER BY
FileType DESC,
FileGroupName,
LogicalName