Question:
I'm trying to install an extension using a .tsep file and I am getting the following error:

"Unable to import the selected extension as the package signature is invalid."
How to avoid this?

Answer:
If a .tsep file is at some point sent as an email attachment, some mail server and/or antivirus software could look into the .tsep file and inadvertently destroy the signature, which makes the .tsep to show this error when double-clicking the file. When the file is distributed in other ways, there is no such problem. 
The workaround to install these invalid .tsep files is to copy them to the \ProgramData\Trimble\Tekla Structures\<version>\Extensions\To be installed folder. When Tekla Structures starts, it checks the available .tsep installers from the \To be installed folder and installs them automatically.

See also
Installing .tsep packages