SketchUp won't start with selected template

That looks like you just typed template. Did you type Sketchup.template?

The capitalization matters:

Sketchup.template

Is not equivalent to

sketchup.template

I can’t post anymore I can only edit a previous post

Sketchup.template
C:/Program Files/SketchUp/SketchUp 2018/Resources/en-US/Templates/Temp01a - Simple.skp

It’s past midnight here so I’m off to bed - thankyou for your replies. I’ve been watching Fox, hope those people are OK in NC.

so what do I do now - clearly this is an error in the Sketchup 2018 installation file. I can’t reinstall 2017 because my key and install code is out of date.
I expect Trimble to fix this problem.

Yes, prayers out for them!

Hope you had a good night’s sleep when you see this!

The value reported for Sketchup.template proves you are getting one of SketchUp’s default templates that is distributed within the executable package. So, this isn’t your imagination, you are really not getting what you attempted to save as default template.

The next step should be to check where your preference files are telling SketchUp to look. These are SharedPreferences.json and PrivatePreferences.json in the folder where DaveR had you look earlier. Json files are structured text, so you can examine them with any plain text viewer or editor (but don’t change anything unless you are confident you know json!).

In the PrivatePreferences.json file, look for a line that says “Templates”: followed by some file path. It should be a simple path with no final file. Let us know what it says (or if it is missing).

In the SharedPreferences.json file, look for a line that says “DefaultTemplate18”: followed by some file path that ends in a specific file. Let us know what it says (or if it is missing).

If these two preference values are missing or disagree as to the lookup path, that is the source of your problem. Exiting SketchUp, rename both preferences files to something that SketchUp won’t load (e.g. append .back to the file names), and then restart SketchUp. SketchUp should create new default versions to replace the missing files. Now try saving your Template as default and see if it sticks. If it does, one or both of these files was defective and causing your problem.

Alas, the technique of renaming them will have abandoned all of your other preference settings as well! It can be tedious to restore them, but at least SketchUp should be healthy.

Edit: reinstalling SketchUp or doing the “Repair” on Windows will not alter or reset your preferences, so if there is a problem with the json files it will persist.