While raging, a Barbarian cannot cast spells.
The Way of the Sun Soul monk's level 3 feature, Radiant Sun Bolt (Sword Coast Adventurer's Guide, p. 131), enables to make ranged spell attacks.
Can a multiclassed barbarian/Sun Soul monk make the ranged spell attack from the Radiant Sun Bolt feature while raging? Or is the Radiant Sun Bolt feature considered a spell, and therefore unusable by a multiclassed Barbarian Sun Soul Monk when raging?
I'm assuming that you're asking whether a barbarian that multiclassed into monk can use the monk subclass feature Radiant Sun Bolt (XGtE p. 35). (Or a monk that multiclassed into barbarian - same thing.)
Rage (PHB p. 48) states the following about spells:
If you are able to cast spells, you can’t cast them or concentrate on them while raging.
The Sun Soul monk's class feature Radiant Sun Bolt states the following:
You gain a new attack option that you can use with the Attack action. This special attack is a ranged spell attack with a range of 30 feet
This ability explicitly uses one of the attacks you gain from using the Attack action. Therefore, you are not taking the Cast A Spell action (PHB, p. 192), and you are therefore able to use the class feature during your rage.
The description of the Way of the Sun Soul monastic tradition states (emphasis mine):
Monks of the Way of the Sun Soul learn to channel their own life energy into searing bolts of light. They teach that meditation can unlock the ability to unleash the indomitable light shed by the soul of every living creature.
That sounds like it would be difficult to do while raging. Hence, it's within reason to rule this out, but since you could easily find an explanation why it is possible while raging (i.e. you trained doing it for so long, using it comes naturally), your DM should definitely tell you about his house rule before you choose the subclass or even start multiclassing into monk/barbarian at all.
While I'm no expert, it seems that Radiant Sun Bolt is a Spell-like ability and therefore not actually casting a spell, and usable during rage. However it's not subject to the barbarian's rage bonus damage since it is Dexterity based, and the rage damage requires a strength based attack
