The shutter is stuck.

@leonardobue7263 the "ERR" message on a Nikon D90 is a general hardware failure warning and really does not identify a particular error. SInce it pops up with your shutter being stuck it could signify that there is a mechanical jam in either the shutter curtain or the mirror charge mechanism. Download the SM from this age Nikon D90 under the Documents header.

First thing you want to do is remove your lens and inspect the shutter blades. Look at the mirror inside the housing. If the mirror is stuck in the "up" position, look directly at the shutter curtain behind it. If those metal shutter blades are bent, misaligned, or popped out of their guide tracks, your shutter assembly is damaged and will need to be replaced. Just remember, do not touch them if they look perfectly flat and aligned.

Next, you will want to check the aperture control lever. Look just inside the left side of the metal lens mount ring. You will see a small metal lever that slides up and down to control the lens aperture. If this lever gets bent and binds against the housing, it will freeze the entire mechanical sequence and throw that ERR code.

If the shutter blades look okay and the mirror is just stuck halfway, the sequence motor itself might be jammed. Check in the SM around page 69 and others. Remove the bottom plastic cover of the camera body. This exposes the bottom of the motor gearbox. You can use a small pick or a precision flathead screwdriver to manually rotate the main sequence gear a few teeth to try and free it up.