Need some more clarity to your post, please provide more relevant information. Source is not sending logs or is it sending and Graylog not receiving? what kind of logs from what kind of device, how is it set up, what is the Input you are using… are you seeing anything in the Graylog logs? Where are you seeing the GROK error coming from?
You can find more on how to ask questions with helpful information here and here. It includes things like posting code with the </> forum tool to make code/logs more readable and how to pull config files without including all the comments (Commented lines starting with #) Also making sure you are obfuscating any private information…
We no nothing about your setup so any relevant information you can provide including steps you have tried are very helpful for figuring out how to solve the incident!