In deployment tasks you can't use placeholders as in action commands. Theoretically in this case you need to exchange
{actionpath} with your full action path and you can leave out
{computer}, as all deployment steps will be executed directly on the target machine(s), other than asset actions, which will be executed on the computer you are using to open the web console. For this reason however you likely don't need to use psexec.exe at all. Just create a Command step and enter the command which you would like to execute on the target machine. We recommend testing commands first on an example machine in order to know that it does what you expected and no user interaction is required.
For details on how to create a deployment package, have a look at
this KB article.