Damp masonry and defective foundation?

Hello,
I am new here in the forum and need help from people who are familiar with this matter.
I have a small allotment garden with damp walls and would like to seal the house from the outside. It is an old building and it looks as if no [Speerbahn] has been installed under the masonry. A horizontal barrier is unfortunately not an option due to cost reasons. My suspicion is that the transition from the foundation to the masonry is not sealed, and I hope that with your help I can get the damp walls under control. I have already started to expose the foundation and plastered the wall. What should I do next? Regards, Timm

By the way, the house is built with [Kalksandstein] and I plastered it with [Kalkzementputz].