SCCM : How to create reports or import templates

"While upgrading my sql server and sql server reporting services to version 19, I lost all my sccm reports. " - do you still have the old environment by any chance or this was rather a inplace upgrade? And from which version did you came to 2019 because here https://docs.microsoft.com/en-us/sql/reporting-services/install-windows/upgrade-reports?view=sql-server-ver15is stated clearly that:

• When you upgrade a Reporting Services installation to a SQL Server 2016 Reporting Services or later (SSRS) installation, existing reports and snapshots that have been published to a report server are compiled and automatically upgraded to the new schema the first time they are processed. If a report cannot be automatically upgraded, the report is processed using the backward-compatibility mode. The report definition remains in the original schema.

So you should be able to retrieve them somehow.

So if this was an inplace upgrade and you did unfortunately uninstalled SSRS in the process - Do you have a backup of the ReportServer database from the older version? You should be able to use that backup and recover your SSRS reports.
As far as how to import SSCM templates you can try do a internet search for "free sccm reports" then download the RDL's for the reports you want and import them into your SSRS however...the option above should be the best for you as you should be able to restore all reports you had prior to the upgrade.