I Have a problem. The Log is 'Couldn't set project path to: C\....'

show me ‘Couldn’t set project path to: C:\…’.

What is this problem. I need your help. Thanks.

This message is only generated when you use the -projectPath command line argument. Unity will tell you if the file path that is given does not represent a Unity project. A Unity project is the folder that contains the Library and Assets folder. The OP has given a path that includes Assets so is one folder too deep.

I have had this same problem for a few days.
I was running the unity in command mode from a bash script and it would fail with the same error.
It turned out that the command line option works a little oddly as I found out.
From a Terminal window the following works just fine:

/Applications/Unity/Unity.app -batchmode -projectPath /Users/you/pathtoproject -quit

However this exact same command line fails when inside a bash script with the error
‘Cannot set project path to /Users/you/pathtoproject’
You need to format the option -projectPath=/Users/you/pathtoproject with an equals sign between -projectPath and the path rather than a space as indicated in the documentation.

Hope this helps if you get stuck when automating builds