When i changed ignition on '01 ford explorer I had to replace steering column&it wont turn over&the theft light flashes&wont stop.The locksmith said antenna dont match Can i reprogram so it'll start?

This is how you do a security system reset on an Impala. I hope you have the transponder receiver connected on the new column.

https://www.vehiclehistory.com/questions/how-do-i-perform-a-chevy-impala-security-reset-93585

Chevy Impala Won't Start Security Light - It Still Runs

https://itstillruns.com > chevy-wont-start-security-light-...
Make sure the security light is flashing. After 10 minutes the security light will stop flashing and either turn off or stay on steadily. When it does, then ...

How to reset anti theft system on a 2008 chevy impala - Fixya

https://www.fixya.com > ... > 2008 Chevrolet Impala
Sep. 1, 2019 - Turn ON the ignition, with the engine OFF. · Attempt to start the engine, then release the key to ON (vehicle will not start). · Observe the SECURITY light, after ..

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recheck connections under dash where you disconnected things to replace column. did the replacement one have the same wiring connectors as the old one.

you need the key fob to start the car

How do you know anti-theft needs to be reset ,Is the security light on or flashing ? Maybe a wiring connector didn't get hooked up correctly .

Passive Anti-Theft System (PATS) Transceiver module is around the key lock cylinder behind or under the plastic cover on the upper steering column'
Passive Anti-Theft System (PATS) Transceiver Removal

1. CAUTION: Electronic modules are sensitive to electrical charges. If exposed to these charges, damage may result.
   Disconnect the battery ground cable, Refer to Section 414-01

1. Remove the ignition switch lock cylinder.
   1. Insert the ignition key into the ignition switch lock cylinder and turn to the RUN position.
   1. Push the ignition switch lock cylinder release tab with a punch while pulling out the ignition switch lock cylinder.
   2. If equipped, remove the tilt wheel handle.
   3. Remove the upper and lower steering column shrouds.
      1. Remove the screws.
      1. Remove the upper and lower steering column shroud.
      2. Remove the lower instrument panel steering column cover.
         1. Remove the screws.
         1. Remove the lower instrument panel steering column cover.
         2. Remove the instrument panel steering column opening cover reinforcement.
            1. Remove the bolts.
            1. Remove the instrument panel steering column opening cover reinforcement.
            2. Remove the anti-theft transceiver module.
               1. Remove the screw from the bottom of the transceiver module.
               1. NOTE: Only apply pressure or leverage below the key cylinder lower rib.
                  Disconnect the electrical connector and remove the module.

The old key may have an induction chip in to deactivate the anti theft system. Does it have a red insert (chip) in the key handle? Get the old key and place it as close to the ignition barrel as possible. Seen this done on a number of occasions to save paying for a key reprogram.

The dealer will need to reprogram it.

you could pull the fuse disabling th anti theft totally.. not sure that would work though

if you disconnect the battery and let it sit awhile this will disable the airbag.

it sounds like either the key is going bad (chip inside) or the PAT's immobilizer system is not detecting a key is inserted with the correct code. If you remove the column surround you will see connectors just make sure all of these are secure, watch the yellow one do not disconnect it as its the airbag and theres a chance of it deploying.
Also try your spare ignition key if you have one.

If all is ok i'm afraid you may need to make a trip to ford for them to diagnose, the pat's system works well almost too well sometimes. i deal alot with them while installing remote starts and when they go wrong its almost always the dealer to resolve it. They have the computer software to test the system.
Sorry,