OS X: Bad substitution

[Nielsbishere]

On OSX (latest) I'm getting the following error (it works fine on windows):

Run git clone https://github.com/DiscordHooks/github-actions-discord-webhook.git webhook
Cloning into 'webhook'...
webhook/send.sh: line 11: ${1,,}: bad substitution
sed: 1: ":a;N;$!ba;s/\r{0,1}\n/\ ...": unused label 'a;N;$!ba;s/\r{0,1}\n/\n/g'
[Webhook]: Sending webhook to Discord...

######################################################################## 100.0%curl: (22) The requested URL returned error: 400

[Webhook]: Unable to send webhook.