That is a correct error report. It is declared upon mesh.ts. Is your TS-based shirt… somehow different than the shirt we are using in the playground?
I know almost nothing about TypeScript, sorry. Keep watching for more replies from TS experts who might know why this is happening.
I remember a little bit about casting from Java programming I did YEARS ago. Have you used “as AbstractMesh” somewhere in your code… where you should be using “as Mesh”?
Another possible “thing”… SceneLoader.Append is really quite old/outdated. I think you should switch to the much newer and nicer AssetsManager. Doing that… might ALSO prevent the shirt from being cast/classed as an AbstractMesh, and instead be cast/classed as a Mesh.
No promises. Just some ideas. 
Stay patient… other forum helpers are nearby and many are experts in TypeScript issues. Sorry I wasn’t more help.