Backup database on local pc

I am using this script to backup database on computer A:

sqlcmd -U MyDatabase -P MyPassword -S .\SQLEXPRESS -Q "EXEC sp_BackupDatabases @backupLocation = 'c:\shared', @databaseName='MyDatabase', @BackupType='F'"

everything works fine, but I decided to store backup files to computer B. I shared folder on computer B, which is accessible from:

\\computerB\shared

I changed backup script to include new backup location:

sqlcmd -U MyDatabase -P MyPassword -S .\SQLEXPRESS -Q "EXEC sp_BackupDatabases @backupLocation = '\\computerB\shared', @databaseName='MyDatabase', @BackupType='F'"

But when I run this, I am getting an error: > Cannot open backup device '\\computerB\shared\MyDatabase_FULL_101 > 22022_232734.BAK'. Operating system error 5(Access denied .). Msg > 3013, Level 16, State 1, Server WIN-C28934ASNF1\SQLEXPRESS, Line 1 > BACKUP DATABASE is terminating abnormally. I don't understand why am I getting Access denied. I can easly eccess \\computerB\shared from file explorer on computer A. What is wrong there?