Problem translating a single document

Hello everyone. I have Studio Starter 2017. When a client sends me a package with a TM inside it works perfectly, but when I try and translate a Single document, the Translation Memory and Document Settings window comes up  and I can choose the languages, but when I click OK nothing happens and I can't move on. Sometimes it does allow me to click OK, but then I get the following error message: "Object reference not set to an instance of an object".

Does anyone have any ideas? Thank you! Louise