Having a Fagradalsfjall volcano tour during an eruption is completely dependent on the tour company operating it. Some companies will not risk taking visitors to the site while others aren't as fussy and will just ensure you keep a safe distance. Ultimately, your visit will also depend on whether the government is advising against travel in the area or not - tour companies cannot overrule scientific bodies' rules and advice. Conditions are closely monitored day-by-day and tours will only go ahead if safe to do so.