This problem arises when a tool requirement cannot be deleted tool from a standard task created from a task template.
Bug # 101290 to provide the ability to delete tool requirement from a standard task created from a task template has been raised to address this issue and will be considered in a future release.
Workaround
Work around would be to break the link between the standard task and template. But, after breaking the link, adding a new resource will remove all other existing resources in that standard task.All resources needs to be added again.
Once the link is broken, resources can be deleted.
SR7000877919