While matching transaction I have the message : Something's not quite right. An unexpected failure occurred. I don't know what goes wrong?

Hi nancy-arraitz,

Thanks for reaching out here.  Matching your transactions in QuickBooks Online ensures a smooth reconciliation of your accounts.  However, the message you're receiving is a known issue for which our Development Team is currently investigating.  Although we don't have an ETA for resolution, I suggest contacting us outside of Community.  We'd be happy to get you set up for an email notification when the issue has been fixed, so you can get back to matching your transactions in a timely fashion.

Schedule a Callback or start a Chat: click (?) Help in the upper right in QuickBooks Online > enter "Contact support" > hit Contact Us > provide a brief summary of your situation > hit Let's Talk > choose Get a callback or Start a Chat.
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Same issue with @nancy-arraitz 
My colleague will reach out to support as advised.