Currently "Sync to Teams" button in SharePoint App Catalog doesn't work or works unstable. Here is a related issue As a result, if you create a new SPFx project based on v1.8 you will not be able to add it to Teams organizational store. You will still be able to do that if you create your own Team's app manifest (it was generated by Yeoman SPFx generator v1.7.1) and create a .zip archive to contain this manifest and icons from teams folder. As the manifest file is no longer generated I decided to provide its content here.
You can take this JSON, replace YOUR_PACKAGE_NAME,YOUR_WEB_PART_ID,TEAMS_APP_NAME,TEAMS_APP_DESCRIPTION, save as manifest.json in teams folder, create .zip archive that contains teams content and upload to Teams Organizational Store. One note: do not create .zip archive in teams folder as it leads to issues when adding SPFx solution to the App Catalog.