Did you add any custom library nodes to the graph? The r.brg may have been included as a dependent library at the time the library node was originally build.

If you are not using an open source R environment for statistical analysis in any of your nodes in the graph then you can remove the reference to the r.brg

I suggest making a copy of the graph first in Windows Explorer so that you can safely examine the effects of the change. Open the graph - the repeated warnings about the missing library brg can be okayed. If a node relies on the missing library then a new warning should then be displayed that indicates which node in the graph has a 'bad type' , e.g.

Within the graph the affected node will be shown on the canvas as a placeholder, e.g.

You can then investigate whether the affected node is relevant to your analysis or can be deleted.

If the R node is not required for your analysis you can remove the r.brg as a dependent library by clicking on:

Graph -> Edit Base Libraries

Then in the Base Libraries dialog select the entry for the r.brg in the list and click the red 'X' delete button,

Okay the dialog and save the graph. Close the graph and the warning messages should no longer be present when you re-open it.