Before creating a Linkedin Messaging ad, you will need to have a selected sender for those messages.
First of all, make sure the sender has both access to the Linkedin Ad Account and to the Company Page.
Secondly, we should add the sender in Linkedin.
To add one of your 1st-degree connections as a sender:
Sign in to Campaign Manager.
In the Campaign Groups tab, select the required campaign group checkbox.
In the Campaigns tab, select the required campaign checkbox.
In the Ads tab, select the required ad checkbox.
If the campaign has no associated ads, click the Create button and select Ad from the dropdown to create a new message ad or conversation ad campaign.
Click the ... More icon to the right of the ad name and select Edit from the dropdown.
In the Edit page, click Add sender.
In the Add Sender dialog, search for the sender that you’d like to add.
Select the sender from the dropdown and click Send request.