Achickenpox virus invaded a cell. inside, the viral genes were expressed and the viral genetic information was transcribed and translated into viral capsid proteins.
what happens when a cell transcribes a viral gene?

The answer is the host cell translates the viral gene.

The central dogma of molecular biology explains that the sequence of DNA specifies the sequence of mRNA, which, further, specifies the sequence of proteins. The same can be used for viral genes. Inside the host cell, the viral gene undergoes transcription. As the result, a viral mRNA is produced. Viral mRNA specifies the sequence of proteins. The host cell's machinery is used for translation of the viral mRNA which will result in viral proteins.