From the error message, it looks like Tiled2Unity is trying to do something illegal in this version of Unity, and so its operation is aborted.
The website for the tool reads:
Warning: Tiled2Unity is old software!
Tiled2Unity was created before Unity had support for tiles and tilemaps. It is still available for download but is outdated and unsupported. Consider using SuperTiled2Unity instead. Thanks!
And the SuperTiled2Unity page elaborates:
SuperTiled2Unity is the second generation of Tiled2Unity, a popular exporter I first released in May of 2014 which is now deprecated.
This means the original version of the tool is no longer supported, and may not perform as desired on more recent Unity versions. I'd recommend using the updated version instead.