I'm trying to add a .pyt toolbox file to the ArcToolbox. When browsing to the .pyt file to add it, it has a red x on the icon. Once selected and Opened, it won't add the toolbox/tools to the ArcToolbox list. It added fine last week.
Using the ArcMap Catalog window (or ArcCatalog itself), browse to the offending pyt (don't use the ArcToolbox -> Add Toolbox dialog for this test).
Does the toolbox have the "X" or the tools inside?
i.e
If it's the toolbox (pyt) - right click on the .pyt and select "Check syntax...".
If it's the tool - right click on the tool and select "Why...".
Post the output (as text, not screenshot).
I also have this question when trying to add my Python Toolbox to ArcMap. ArcMap is using Python 2.x, so that it's a bit different from ArcGIS Pro and there is also some strict limit. I finally fixed this issue by checking two aspects: Syntax and Encoding.
1. Syntax Errors
Please make sure there are no syntax errors before adding to ArcMap.
2. Encoding
Please make sure the encoding of your *.pyt is correct (Python 2.x needs you to define the encoding). You can use all English characters or adding an encoding at the header of the file.