The problem

You try to mount the VMware tools ISO but you get the following error.

Operation "Mount VMware Tools." failed for VM "MyComputer" for the following reason:
The operation is not allowed in the current state.

Using the following command line to remove any existing ISO does not solve the problem:

Get-VM -Name MyComputer | Get-CDDrive | Set-CDDrive -NoMedia

Restarting the computer does not solve the problem neither.

The solution

What worked for me was to stop the VM and start the VM again.