Are the underlined verbs used in the right forms in the following sentences?
1.We were to meet at seven, but I wasn't able to come there in time. I think he should have waited.
2. She guessed she must be having an allergic reaction, but she had no idea what she was reacting to.
3. I had no accurate idea of the time, for I couldn't see my watch, but I thought we had to start about four hours ago.
4. You mustn't (couldn't) have asked for anyone more regular than Peter.
5. They must have thanked us for all we have done for them.
6. He behaves as if nothing has happened. But he forgets it is very important tha he will take a chance of going there.
4 is correct with "couldn't
3, 5 and 6 are not necessary incorrect but the meaning is not very clear. I suggest you try rewriting them to make your meaning more precise. I suspect you are trying to say something a bit different from what appears here.
For instance, in number 6 you might be saying:
But he forgets that it is very important that he takes a chance (that is, a risk) and goes there.
Or you might be saying,
But he forgets that it is very important that he avails himself of the chance (that is, the opportunity) to go there.
Again, I would try writing these one more time with different wording.