Close the XAML
Close UiPath / Project
Take a backup from the XAML
Open the XAML within a text editor (e.g. notpad++)
Add the Assembly reference in case of it is missing:
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<AssemblyReference>System.Security.Cryptography.Xml</AssemblyReference>
Save the file
Open studio and try again. Now the Class and namespace should be recognized
Maybe do the same for <AssemblyReference>System.Security.Cryptography</AssemblyReference>
and ensure that the invoke code is correct and do not use unneeded / unsupported statements