Sometimes we may get this error whenever we re-install the workflow after doing some changes. It may come once you un-install the workflow and install it again.
Now how to allow new instances for workflow?
To resolve this problem we need to set the workflow association to "Allow" and the new instances of the workflow should be created again. In order to set this option, what we need to do that, go to the Remove Workflows settings page by List à Settings à Workflow Settings à Remove Workflows.
And here we need to select the “Allow” option.
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where i have to find this page in sharepoitn 2010