Straight-up sugar, as in sugarcane, is essentially an early modern innovation in European cuisine. In the very rare case it's known in the Middle Ages, it's lumped in with spices ("something rare and expensive from far away") and generally treated as a medicine to be taken in tiny amounts. The classic account of the rise of sugar is Sidney Mintz's Sweetness and Power: The Place of Sugar in Modern History, if you are interested in that part of the story.

That said, there are definitely ways that sweet taste was integrated into medieval eating and culture. The two biggest are, predictably, fruit and honey.

The scope of fruit consumption is somewhat awkward to study for the Middle Ages. We hear quite a lot about gardens in sources. But when it comes to household registers (money management stuff, basically) and recipes, things grown "in-house" aren't typically mentioned. This has led some scholars to question whether nobles ate any vegetables at all! On the other hand, fruit was definitely being produced for market. In the mid-13th century, the bishop of Winchester had a garden-orchard at one of his estates that had, together, 129 apple and pear trees. Viticulture was also huge in medieval Europe--sources seem to indicate that more of western Europe was suitable for growing grapes in the high Middle Ages than today, even. Although we typically think of vineyards in the context of wine and verjuice (the medieval version of vinegar/citrus/other acid for cooking), it's quite clear in literary sources that people enjoyed grapes by the bunch on their own. Strawberries are the other fruit often given a mention--in fact, strawberries and their vines even appear as a decorative motif in late 14th century medieval manuscripts (before fading away).

By the later Middle Ages, as intra- and intercontinental trade really picked up, citrus fruits became a little more widely available in Europe for those who could afford them, particularly in Italy. I guess we don't really count lemons as sweet today, but I think oranges might qualify.

The other major source of sweetness was honey. Honey actually played a role in European culture beyond food. Since it's the biblical sweetener, and always referenced in a good way, there was a fairly strong association between sanctity, sweetness, and honey. Saints' bodies smell like honey, or honey flows from Christ's mouth into the poetic narrator as a material symbol of his love.

But it was also a key component of medieval food. I'm going to pick on Nuremberg here as an example. Nuremberg's forested territory outside the city walls, gradually acquired for itself from the emperor over the 14th and 15th centuries, contained substantial amounts of monitored beehive. Production of honey, in and of itself, was a major product both in and out of the city. But where Nuremberg stands out is its famous use of honey in baking. Specifically, its Lebkuchen or honeycakes, and related Pfefferkuchen--basically the forerunners of modern gingerbread. Nuremberg was definitely not the only city making honeycakes in significant amounts. In fact, the earliest reference to a baker specializing in Lebkuchen alone comes from Munich. But Nuremberg was quite famous for them.

Tragically, there was no way for medieval European to eat my preferred diet of nothing but ice cream, although in the era before toothpaste and fitness tracker apps, perhaps this was not such a bad thing. Sugar was exotic or unknown, but sweetness was not.