Its a consequence of the row over the Northern Ireland Protocol.
EU exit and the Northern Ireland Protocol
It was signed and agreed as part of the main Withdrawal Agreement. It has the status of an international treaty. However, now that the UK has left the EU, the government (that signed the Protocol) have decided they don't like it, and unilaterally want to change it. This hasn't gone down well with the EU (which is understandable), and one of the consequences has been that the UK's participation in the EU's "Horizon Europe" program has been delayed until the issue is resolved. I would imagine that if the UK Government decide to proceed with their unilateral action, the UK's participation in the Horizon program will be terminated.
I suppose the simple message is; if you sign an international agreement, it may be best to keep to it if you want to maintain good relationships!