Hi there, baltack.
It appears that you signed in on your iPhone with a different Apple ID. Now you've received a messages that says, "This device is associated with another Apple ID. You cannot associate this device with another Apple ID for 90 days.", and I'm glad to help.
View and remove your associated devices in iTunes -- This article provides the information that you need regarding associated devices.
Apple - Support - Product Selection — If you'd like, you can contact Apple Support to see if the time limit can be removed. After selecting this link, choose Apple ID > Other Apple ID topics > The topic is not listed. Type "associated devices" in the field provided. Then select how you'd like to start the conversation.
Take care.