Slow Time Club is made for iPhone, running iOS 18 or later. There's no Android version.
Check your iOS version
Go to Settings → General → About and look at iOS Version.
If it's older than 18, update it from Settings → General → Software Update.
Other Apple devices
Your blocks live on the iPhone where you set them up. They don't carry over to an iPad or Mac, even with Share Across Devices turned on in Screen Time, because that setting only shares Apple's own Screen Time limits.
If you'd like limits on another device too, Apple's built-in Settings → Screen Time can set them there.
Work, school and child phones
Some iPhones are managed by an employer or school, or belong to a child in Family Sharing. On those phones, Apple or the organisation may limit which apps can use Screen Time. If Slow Time Club can't get access, ask whoever manages the phone.