Hi @FaRoss Wright ,
Since we can't reproduce your situation, we need to narrow down the problem and try some things.
Please try the following and let me know the results:
1.Remove your currently permission,then assign Full Control permission directly to the affected user account.
2.Clear the cache of the SharePoint Designer.
3.Open SharePoint Designer and connect to the site collection again, when it’s prompting to input the login account, make sure the Keep me signed in is checked.
Addnitionly, if none of the above attempts work please check this article:https://answers.microsoft.com/en-us/msoffice/forum/all/sharepoint-designer-2013-server-error-access/63a3358c-3722-49b6-a15e-e7a1dfa2e3c1
The issue you met often occurs when a particular permission is not included in FULL CONTROL, and even if you are an admin you cannot edit or create a workflow for a site via SharePoint designer 2013.
To fix, you need to follow the steps from the link below:
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