Hello @Cornel Verster and welcome to Microsoft Q&A.
Before I get into trying to help, I noticed a very similar question asked by you:
https://learn.microsoft.com/en-us/answers/questions/466038/index.html
Would it be acceptable to merge these two?
----------
Okay, so the pipelines previously ran fine. You did test connection on datasets and that checked out. Other investigations you made also looked good.
So far there are not many things I can think of to help, but I will try my best. If I cannot help, I offer you a support ticket.
Did the error message show the expected table name and schema?
Did you do a deployment recently? Perhaps something changed the Linked service.
If the Linked service is pointing to a real but incorrect database (like dev / test), I could imagine not finding the table.
Please try using the "preview data" feature in the dataset or copy activity. I find "preview data" to be a much better functionality test than the "test connection", because "test connection" doesn't actually try to get data.