they will pick up serpents with their hands; and if they drink any
deadly poison, it will not hurt them; they will lay their hands on the
sick, and they will recover.” (Mark 16:18 - ESV)
Pick up serpents and if we drink poison it will not hurt us, in same vers is healing the sick by laying of hands the sick shall recover. Jesus is talking about power over heavenly beings hear on earth.
Behold, I have given you authority to tread on serpents and scorpions,
and over all the power of the enemy, and nothing shall hurt you.
(Luke 10:19 - English Standard Version)
We have been given power and authority over fallen entities. Satan and his fallens angels symbolized as serpents and scorpions to symbolize demons.
The Sons of Sceva in Acts 19:11-20 were not authorized to cast out demons and because of this they were beaten and humiliated Acts 19:16d "they fled out of that house naked and wounded.
And they were all amazed, and spake among themselves, saying, What a
word is this! for with authority and power he commandeth the unclean
spirits, and they come out. (Luke 4:36 - KJV)
With same authority Jesus rebuke unclean spirits in Luke Ch 4 the very same authority have we been given by him as mentioned in Luke Ch 10.
Also in 1 John 4:4 we can see the spirit of anti-Christ can't affect those with the Spirit of God.
Ye are of God, little children, and have overcome them: because
greater is he that is in you, than he that is in the world. (1 John 4:4 - KJV)
Spirit of God vs the spirit of anti-Christ (1 John 4) He that hath the Spirit of God has overcome this world and can both stand against and rebuke the entities from the dark-principality that satan has under himself.
A rebuttal to the problem with discrediting Mark 16:9-20 by lifting the issue that some manuscript (Sinaiticus & Vaticanus) have a shorter version of Mark 16 that ends at vers 8, it is not a valid argument to do away with Mark 16:18 because the very same account made by Jesus is repeated in the next book, that is in the Gospel of Luke chapter 10 vers 19 in same manuscript (Sinaiticus & Vaticanus) that excludes Mark 16:9-20 does include it in Luke 10:19!
ϊδου δεδωκα ϋμιν την εξουϲιαν του πατειν επανω οφεων και ϲκορπιων και επι παϲαν την δυναμιν του εχθρου και ουδεν ϋμαϲ αδικηϲει
Behold, I have given you authority to tread on serpents and
scorpions, and over all the power of the enemy, and nothing shall
in any way hurt you. Codex Sinaiticus - Luke 10:19
Codex Vaticanus - Luke 10:19

Also something that has to be mentioned in this issue is that a huge empty space occur in Codex Sinaiticus & Codex Vaticanus were Mark 16:9-20 should have been but removed/excluded, this is the only empty New Testament column in these Codex.
CODEX SINAITICUS - Mark 16

CODEX VATICANUS - Mark 16

Manuscript that includes Mark 16:9-20;
- Codex Alexandrinus.
- Codex Bezae.
- Codex Gigas.
Latin Vulgate (382 A.D.) include both Mark 16:18 and Luke 10:19
serpentes tollent et si mortiferum quid biberint non eos nocebit super
aegrotos manus inponent et bene habebunt. Latin Vulgate - Mark 16:18
ecce dedi vobis potestatem calcandi supra serpentes et scorpiones et
supra omnem virtutem inimici et nihil vobis nocebit. Latin Vulgate - Luke
10:19