Error: Forbidden (after entering API key)

I’m getting the following error message:

Error type: “Bad Request”. Error message: “Bad Request” Domain: “usageLimits”. Reason: “keyInvalid”.

Did you added your own Google API key? Look at the help.

Check in YouTube if the id PLMHJAhqaiml3Rb43DpM3Mvyao4fW3TWiR belongs to a playlist. Check the FAQ of the plugin or send error messages to support.

I’ve added the API key as the video suggests. But I’m still getting the error.

Here is the shortcode I’m using:

[Youtube_Channel_Gallery feed="playlist" user="PLMHJAhqaiml3Rb43DpM3Mvyao4fW3TWiR" key="AIzaSyA-_6eKYS6gZwDzlc3hLIDVsmZxGeyTD1g" videowidth="90%" ratio="16x9" theme="dark" color="black" autoplay="0" rel="0" showinfo="0" maxitems="16" thumbwidth="43%" thumbratio="16x9" thumbcolumns="2" title="1" description="0" thumbnail_alignment="top" descriptionwordsnumber="6"]

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Can you help? I’ve never had as many problems with a plugin before as I’m having with this one 🙁 And this makes me sad because this is one my favourites.

https://wordpress.org/plugins/youtube-channel-gallery/