Resume a LinkedIn campaign (ad set)

Sets a campaign (an ad set in Campaign Manager) to ACTIVE so it can deliver. Real money starts being spent as soon as LinkedIn approves the creatives. Use when: the user has reviewed something paused — usually a campaign this server created — and wants it live. Do not resume without telling the user the daily budget first, and without checking three things that quietly keep an ACTIVE campaign at zero: the campaign group (the Campaign Manager campaign) must also be active, at least one creative (ad) must exist and be resumed, and LinkedIn must have approved it. linkedin_get_campaign_structure and linkedin_list_creatives answer all three. Do not report "resumed" as "running". Check the read-back: the serving issues it returns are LinkedIn saying why nothing is delivering. A campaign whose end date has already passed is refused rather than resumed: LinkedIn would accept it and serve nothing. Give it a new end date with linkedin_update_campaign first. Returns the campaign read back, including its budget and any serving issue.