First, check the manufacturer's installation instructions. The length of most portable A/C exhausts is limited and it may not be sufficient to reach your attic. Assuming it will, the concern about venting it into the attic is the potential to introduce moisture and the attendant problems (e.g., mold, mildew, etc.) You might consider boosting the flow of conditioned air to the room either via changes to your existing duct work or using a transfer grill and fan as alternatives. Either would enhance cooling without the need for an exhaust or the need to deal with condensate.

Thanks a lot Charles. Much appreciated. I will put the ac on a secure shelf so I will have enough length. I was thinking the main ac unit will take care of the moisture in home. Also the portable ac has dehumidifier with a hose that can gather water in bucket. Do you still think there will be additional moisture in attic? Thanks again for the response.

Provided your unit produces condensate, you'll be introducing some amount of moisture into your attic in the exhaust air stream. Whether that causes a problem depends on the rate at which moisture is introduced, the attic volume, and the attic ventilation rate.

You may be able to address your issue less expensively--and without any condensate or exhaust stream to deal with-- if you use a powered room-to-room transfer fan. Here's an example:

https://shop.tjernlund.com/products/as1

No you may not exhaust anything into an attic.

You may pass through the attic to the exterior though.

I agree it's not good idea to exhaust anything into an attic and I think the objective can be better achieved without the added cost and complexity of a portable A/C unit. That said, I don't believe that venting the OP's portable A/C into an attic is prohibited by code--at least not the 2018 IRC, but, it might be buried somewhere in the fine print.

Thanks Charles and others! Noted! I will look for another option. Thanks a lot for the input

Portable air conditioners are a poor way of cooling and in your case I believe it will be detrimental to overall cooling of building. Your portable obivously doen't have both intake and a discharge hoses or you would be discharging at same place the intake comes from outdoors. Using room air to cool coils then discharging (to attic or outdoors) heated air does 2 negative things. #1 Discharging air from inside a building will pull an equal quinity of air from outdoors. You just took xx cubic feet of cool air and exchanged it for hot air which now has to be cooled. #2 Not only did you trade cool air for hot air, the portable motors give off almost as much heat as the cooling coil takes from air. So you actually add heat to inside building instead of cooling it. You are 10x to 1 better off doing as Chas Ross suggested in moving air from adjoining room(s) to computor room. Just for the record even though it is the least efficient means of cooling, a portable with intake and discharge hoses can cool a room better than a funeral fan if no other options are available. Bottom line, a 120 volt window shaker is a far better option than a portable.