Here are the situations when an activity can appear as missing or invalid:
There are two ways of verifying the root of the problem, checking the logs panel and checking the project's dependency tree.
Use this solution for verifying if the required package version corresponds to the installed one.
The easiest way of checking the errors displayed in the workflow is by examining the Output panel situated at the bottom of your UiPath Studio framework.
All errors are displayed in red and by reading the error you can easily identify the affected package.
The GIF below shows that the missing activity is named Get PDF Page Count and is part of the UiPath.Pdf.Activities package.
More information about Managing Dependencies can be found here.
Another solution for solving the displayed errors is by verifying the project's dependencies. By selecting the project's root, you can see all packages and their dependencies included in the project.
The first step is to check if the package mentioned by the error is part of the project, and if so, to verify its version, and update it to the required one.
The below GIF explains how to check the dependencies.
A dependency can be invalid or missing.
More information about Studio Logs can be found here.
After applying one of the above steps for identifying the problem, you can go ahead and fix it by installing a newer package or package version. All you need to do is to access the Manage Dependencies option and search for the desired package.
An example is shown in the below GIF:
Updated 4 months ago