can SOT883B package BJT transistor switch a relay with 21.1mA coil current?
The link to the relay is broken, but it's for you to look up anyway.

You already know the transistor can handle 100 mA, so the question is whether the relay will require more than that.  You say the relay only requires about 21 mA, which is obviously less than 100 mA by a good margin, so I don't see what you are actually asking.

You say you'd be powering the relay from 3.8 V.  The transistor can handle 45 V, so that is clearly within its capabilities.  That is again so obvious it's hard to see what you are actually asking.

You have to make sure that your relay can really operate from 3.6 V (the transistor will eat up about 200 mV when on) coil voltage, and that it really does draw less than 100 mA when it does.  This again should be obvious, coming directly from the relay datasheet.

Two things that are less obvious to watch out for are: 1 - make sure to put a reverse polarity diode across the relay coil.  2 -  Make sure the transistor can dissipate the power.  That should be a no-brainer if all the current thru it comes when it is saturated.  Since that can't be more than 100 mA and saturation is probably (your job to check) 200 mV or less, it can't dissipate more than 20 mW in that case.