This error usually happens when the project does not contain a reference to the Telerik.Web.UI assembly, or the reference is broken. Please refer to the following articles which provide possible solutions for the observed problematic behaviour:
Unrecognized tag prefix solved 1
Unrecognized tag prefix solved 2
Unrecognized tag prefix solved 3
Unrecognized tag prefix solved 4
I have listed so many help posts since I am not aware of your specific scenario. I hope they will prove helpful.
the Telerik team
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