The Romans - 30:34
So they are ungrateful for what We have given them. So enjoy yourselves; you will know in the future.
In the context of verses 33,34,35, ingratitude to Allah; taking gods who have no power or authority beside Him, and serving them, while Allah is the One Who has given everything we know at our command:
The Bee/16:54
But when He removes that distress from you, a group of you immediately begin to associate partners with their Lord,
The Bee/16:55
Ungrateful for what they were given Enjoy yourselves; soon you will surely know!
The Night Journey/17:67
If any distress touches you in the ocean, those whom you call upon besides Him are lost. But when He saves you and brings you ashore, you return. Man is ungrateful
The Spider/29:65
When they board the ship, they call upon Him, making the religion belong to Allah alone. But when We deliver them from the sea, they start associating partners again.
The Spider/29:66
Let them be ungrateful for what We have given them and enjoy themselves. They will know in the future.
The Romans/30:33
On the Day when the people are afflicted, they turn to their Lord in prayer. But then, when they taste a mercy, some of them associate partners with their Lord.
The Romans/30:34
So they are ungrateful for what We have given them. So enjoy yourselves; you will know in the future.
The Companies/39:3
"Indeed, to Allah alone belongs religion. Those who take those other than Him as awliya (friends) say: "We serve them so that they may bring us closer to Allah". Allah will decide between them in that in which they differ. Surely Allah guides not the liars and the ungrateful."
The Forgiver/40:42
You call me to be ungrateful to Allah and to associate with Him that of which I have no knowledge. I call you to Him Who is the Exalted and the Forgiving.
The Ornaments of Gold/43:15
They set aside some of their servants as a portion for Him. Indeed man is a manifest ingrate.
The Ornaments of Gold/43:16
Has He reserved for Himself daughters from among His creatures, but has He chosen sons for you?
The Heights/7:189
"He created you from a single soul (with the same genetic characteristics). And from him He created his mate so that he might find rest. When he approached his mate, he loaded her with a light burden and traveled with her. When his burden became heavy, they both begged their Lord, Allah: "If you give us a perfect child, we shall be among the thankful."
The Heights/7:190
When He gave them a perfect child, they began to associate partners with Him for this gift He had given them. Allah is far greater than all that they associate partners with.