I searched about your problem and this was also asked in Turkish language in google chrome community.
The package is invalid. Details: 'Variable $1$ used but not defined.'I just used the google translate for the thread and one recommended answer there in green said that it is the apps problem and will need to wait for its developer/publisher to fix it. The source of the answer was also on that link and I just also translated it. You can check it for reference since the thread was just also posted 19 hours ago as of typing.
Thanks for your help. It was the exact same error and they said it's a problem on metamask's end. Now it's fixed I could install it without a problem.
Don't mention it. It's great that it has been fixed by the publisher just as that guy have answered on that thread. Well, it seems that this thread has already served the purpose and you're able to download what you're trying to download. Maybe for future reference, someone might find this thread and will refer if ever the same error exists as they download some extension. You can go ahead and lock the topic as it's solved your concern.