This means that TMS can not find all instances of a specific metafield key.
However, we can use the following workaround to translate a specific metafield, based on keywords that are shared between all the values.
Lets say we have the following product metafield, with all of the text in the box below as the value.
We don't have the key, but lets try to find some keyword in this metafield, that will be on all the other metafields.
The keyword "🚀 Shipping time:" seems to be in every product shipping metafield.
We can now go to TMS -> "Manual Translation" -> "Find by keywords search",
By searching for the keyword, we can find every instance of the metafield and translate them easily.