Remove both termbases from your project and add them again.

Hi Jesus

The trick worked and now I can choose my desired TB! But why switch to turn and make default a TB in the list is a bit weird. I should select the other TB, to let me make default the required one I need to use in my translation and add term into it. Why at first it doesn't allow me to select the TB I need without select the other one? 

I guess you moved your termbases from one folder to another, so Studio couldn't connect to them anymore (that's the reason of the red icon).

When you deleted them and added them back, the connection was re-established so you can operate with the termbases as usual.

I moved them to the same folder and now I ask the question about selection, both TB is in the same folder, not in different folders. I know what you say about moving and breaking the link to a previous place that understandable for Trados. for example, currently I deleted one of TB and just remain a TB in the list, but the "default" button is grey out, and need to close the window and back to see it turn into default! is it a bug? 

There may be some reasons. For example, maybe you open the project first and then moved the termbases to the right folder. In this case, you might need to close the Termbases dialog box and re-open it, because the status of each termbase is not refreshed at every second. This is just an example, I don't know if this is the real case here.

My tip would be that if it's working now, just let it be. If it fails again, then get back to the community.

Glad it worked!

There may be some reasons. For example, maybe you open the project first and then moved the termbases to the right folder. In this case, you might need to close the Termbases dialog box and re-open it, because the status of each termbase is not refreshed at every second. This is just an example, I don't know if this is the real case here.

My tip would be that if it's working now, just let it be. If it fails again, then get back to the community.

Glad it worked!