How to send parameters to BCP inside dynamic sql using sp_executesql
I think you'll have to externalize those parameters when generating the dynamic SQL.  Something like:

DECLARE @REPORTID Nvarchar(500) = 'SomeID_Resend2019-02-20 11:38:24.040';
DECLARE @Path varchar(500) = '\\server\folder\_FileName.csv';

DECLARE @SQL nvarchar(500);

SET @SQL
    = N'master.dbo.xp_cmdshell ''BCP "SELECT RowValues FROM ServerName.dbo.TableName WHERE ReportId = ''''' + @REPORTID + '" queryout ' + @Path + ' -t, -T -N -S SERVERNAME\INSTANCENAME''';
PRINT @SQL
--EXEC sp_executesql @SQL




The PRINT statement reveals the generated dynamic SQL

master.dbo.xp_cmdshell 'BCP "SELECT RowValues FROM ServerName.dbo.TableName WHERE ReportId = ''SomeID_Resend2019-02-20 11:38:24.040" queryout \\server\folder\_FileName.csv -t, -T -N -S SERVERNAME\INSTANCENAME'