You cannot currently connect to a GPU due to usage limits in Colab. – Connect without GPU

What are the usage limits of Colab?link
Colab is able to provide resources free of charge in part by having dynamic usage limits that sometimes fluctuate, and by not providing guaranteed or unlimited resources. This means that overall usage limits as well as idle timeout periods, maximum VM lifetime, GPU types available, and other factors vary over time. Colab does not publish these limits, in part because they can vary over time.

https://research.google.com/colaboratory/faq.html#usage-limits

www.reddit.com : Can’t connect to GPU backend
https://www.reddit.com/r/GoogleColab/comments/1awijw4/cant_connect_to_gpu_backend/

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alternative:
Digitalocean > Paperspace
https://console.paperspace.com

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7 of the Best Alternatives to Google Colab For 2024 (With Free Compute!)
https://saturncloud.io/blog/7-of-the-best-alternatives-to-google-colab-for-2023-with-free-compute/