Must declare the scalar variable error in SQL stored procedure

I am trying to write my 1st of many Stored Procedures which will perform functions on a MS SQL database (source) and a remote linked MS SQL database (target) such as :

• Check Specific Databases exist on Source/Target Servers 
• Check Specific tables exists on Source /Target databases
• Check Tables have identical formats/fields on Source/Target databases
• etc..

The 1st one that I have tried to develop is a stored procedure which will validate that a specific database exists on either Source/Target server. 

CREATE OR ALTER PROCEDURE Tios_Check_DB_Exists
(@linkedServer nvarchar(max),
@database nvarchar(max),
@rowCount int OUTPUT,
@sql nvarchar(max)=' ')
AS
BEGIN
SET NOCOUNT ON;
SET @sql='select @rowCount = Count(*) from '+@linkedServer+ '.master.dbo.sysdatabases where lower(name)=lower('''+@database+''')'
Print @sql
exec (@sql)  
RETURN @rowCount
END
GO

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Calling the stored procedure

BEGIN
DECLARE @rowCount int
exec @rowCount = [XXX\SQLSERVER2019].master.dbo.Tios_Check_DB_Exists '[XXX\SQLSERVER2019]','DTA_TESTDTA' ,-1
print @rowCount
END

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Result

select @rowCount = Count(*) from [XXX\SQLSERVER2019].master.dbo.sysdatabases where lower(name)=lower('DTA_TESTDTA')
Msg 137, Level 15, State 1, Line 62

Must declare the scalar variable "@rowCount".
-1

Completion time: 2022-01-01T20:17:52.9290824+00:00

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Why are I getting the above error when I have defined @rowCount ?

How can I return any error messages encountered ?

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