If Simulink® is unable to find either the library block or the source library on your MATLAB® path during linked block update, the link becomes unresolved. Simulink changes the appearance of these blocks.
Simulink tries to help you find and install missing products that a model needs to run. If you open a model that contains built-in blocks or library links from missing products, you see labels and links to help you fix the problem.
Blocks are labeled with missing products (for example, SimEvents not installed)
Tooltips include the name of the missing product
Messages provide links to open Add-On Explorer and install the missing products
For unresolved library links, double-click the block to view details. Click the link to open Add-On Explorer and install the product.
To fix an unresolved link, you can:
Double-click the unresolved block to open its dialog box (see the Unresolved Link block reference page). If a product is missing, click the link to open Add-On Explorer and install the product. Alternatively, correct the path name in the Source block field and click OK.
Delete the unresolved block and copy the library block back into your model.