After the last Plot statement you are missing a semicolon. This means the Plot and checktemp are multiplied, so for that expression the head is Times, so you are trying to set that expression to a number, rather than the symbol checktemp. This may work:

For[i = 1, i <= 1, i++, 
 sol = NDSolve[{eqR1, eqR2, R[(i - 1)*dt] == Last[Ro], 
    R'[(i - 1)*dt] == Last[Rpo], m[(i - 1)*dt] == Last[mo]}, {R[t], 
    m[t]}, {t, (i - 1)*dt, i*dt}];
 Rbubble[t_] = R[t] /. sol;
 Plot[Rbubble[t], {t, (i - 1)*dt, i*dt}] BubbleRate[t_] = Rbubble'[t];
 Plot[BubbleRate[t], {t, (i - 1)*dt, i*dt}] mdot[t_] = m[t] /. sol; 
 checktemp = Last[Rbubble[(i)*dt]];
 Plot[mdot[t], {t, (i - 1)*dt, i*dt}] ]

Note that I moved the assignment to before the Plot statement, so the Plot appears without a terminating semicolon. A semicolon suppresses output. This means the earlier Plot statements generate no output. If you want to see the plots you can save them as variables and use Show later, or you could enclose them in Print[] for immediate display.

Best, David

Hi David,

Thank you for your help. I understood the issue and corrected it appropriately in my code. It works fine although this has led me to a different problem.

Question - Can we reassign a variable in a loop?

In[2595]:= For[i = 1, i <= 1, i++, 
 sol = NDSolve[{eqR1, eqR2, R[(i - 1)*dt] == Last[Ro], 
    R'[(i - 1)*dt] == Last[Rpo], m[(i - 1)*dt] == Last[mo]}, {R[t], 
    m[t]}, {t, (i - 1)*dt, i*dt}];
 Rbubble[t_] = R[t] /. sol;
 Plot[Rbubble[t], {t, (i - 1)*dt, i*dt}]
  BubbleRate[t_] = Rbubble'[t];
 Plot[BubbleRate[t], {t, (i - 1)*dt, i*dt}]
  mdot[t_] = m[t] /. sol;
 Plot[mdot[t], {t, (i - 1)*dt, i*dt}]
   checktemp = Last[Rbubble[(i)*dt]]
 ]
checktemp = Last[Rbubble[(i - 1)*dt]]

During evaluation of In[2595]:= Set::write: Tag Times in 0  is Protected. >>

Out[2596]= 0.0000500319

The assignment of checktemp inside the loop gives an error, however, a similar assignment outside the loop works fine. How should I overcome this?

Question 2: In the code above, I am extracting the value from function Rbubble using "Last". Is there a better way to get the value as an output rather than a list?

Please help!

Thanks.

P.S. - I am not sure if my work allows me to share those equations hence, I just uploaded the code essential to the issue. Sorry for the inconvenience.